No matter what size the schematic is, the game would crash leaving me with this error every time.

---- Minecraft Crash Report ----

WARNING: coremods are present:
Contact their authors BEFORE contacting forge

// Daisy, daisy...

Time: 6/24/19 9:01 PM
Description: Unexpected error

java.nio.BufferOverflowException
at java.nio.Buffer.nextPutIndex(Buffer.java:521)
at java.nio.DirectIntBufferU.put(DirectIntBufferU.java:297)
at net.optifine.render.VboRegion.drawArrays(VboRegion.java:299)
at net.minecraft.client.renderer.vertex.VertexBuffer.func_177358_a(VertexBuffer.java:50)
at com.github.lunatrius.schematica.client.renderer.chunk.container.SchematicChunkRenderContainerVbo.func_178001_a(SchematicChunkRenderContainerVbo.java:28)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.renderBlockLayer(RenderSchematic.java:802)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.func_174977_a(RenderSchematic.java:793)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.renderWorld(RenderSchematic.java:352)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.renderSchematic(RenderSchematic.java:278)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.onRenderWorldLast(RenderSchematic.java:247)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_22_RenderSchematic_onRenderWorldLast_RenderWorldLastEvent.invoke(.dynamic)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90)
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182)
at net.minecraftforge.client.ForgeHooksClient.dispatchRenderLast(ForgeHooksClient.java:181)
at sun.reflect.GeneratedMethodAccessor56.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.optifine.reflect.Reflector.callVoid(Reflector.java:660)
at net.minecraft.client.renderer.EntityRenderer.func_175068_a(EntityRenderer.java:1951)
at net.minecraft.client.renderer.EntityRenderer.func_78471_a(EntityRenderer.java:1613)
at net.minecraft.client.renderer.EntityRenderer.func_181560_a(EntityRenderer.java:1384)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1118)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:397)
at net.minecraft.client.main.Main.main(SourceFile:123)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Thread: Client thread
Stacktrace:
at java.nio.Buffer.nextPutIndex(Buffer.java:521)
at java.nio.DirectIntBufferU.put(DirectIntBufferU.java:297)
at net.optifine.render.VboRegion.drawArrays(VboRegion.java:299)
at net.minecraft.client.renderer.vertex.VertexBuffer.func_177358_a(VertexBuffer.java:50)
at com.github.lunatrius.schematica.client.renderer.chunk.container.SchematicChunkRenderContainerVbo.func_178001_a(SchematicChunkRenderContainerVbo.java:28)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.renderBlockLayer(RenderSchematic.java:802)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.func_174977_a(RenderSchematic.java:793)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.renderWorld(RenderSchematic.java:352)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.renderSchematic(RenderSchematic.java:278)
at com.github.lunatrius.schematica.client.renderer.RenderSchematic.onRenderWorldLast(RenderSchematic.java:247)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_22_RenderSchematic_onRenderWorldLast_RenderWorldLastEvent.invoke(.dynamic)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90)
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182)
at net.minecraftforge.client.ForgeHooksClient.dispatchRenderLast(ForgeHooksClient.java:181)
at sun.reflect.GeneratedMethodAccessor56.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.optifine.reflect.Reflector.callVoid(Reflector.java:660)
at net.minecraft.client.renderer.EntityRenderer.func_175068_a(EntityRenderer.java:1951)
at net.minecraft.client.renderer.EntityRenderer.func_78471_a(EntityRenderer.java:1613)

-- Affected level --
Details:
Level name: MpServer
All players: 1 total; [EntityPlayerSP['Dannythebrill13'/55, l='MpServer', x=3.52, y=69.97, z=38.75]]
Chunk stats: MultiplayerChunkCache: 800, 800
Level seed: 0
Level generator: ID 01 - flat, ver 0. Features enabled: false
Level generator options:
Level spawn location: World: (45,4,60), Chunk: (at 13,0,12 in 2,3; contains blocks 32,0,48 to 47,255,63), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 3162 game time, 3162 day time
Level dimension: 0
Level storage version: 0x00000 - Unknown?
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: false
Forced entities: 25 total; [EntityPig['Pig'/0, l='MpServer', x=-25.78, y=64.00, z=17.26], EntityHorse['Horse'/1, l='MpServer', x=-45.85, y=64.00, z=76.83], EntitySheep['Sheep'/2, l='MpServer', x=-37.50, y=64.00, z=71.50], EntitySheep['Sheep'/3, l='MpServer', x=-39.50, y=64.00, z=68.50], EntitySheep['Sheep'/4, l='MpServer', x=-35.50, y=64.00, z=66.50], EntityPig['Pig'/5, l='MpServer', x=-30.57, y=64.00, z=14.91], EntityPig['Pig'/6, l='MpServer', x=-22.70, y=64.00, z=21.24], EntityPig['Pig'/7, l='MpServer', x=-24.76, y=64.00, z=10.57], EntitySheep['Sheep'/8, l='MpServer', x=-20.78, y=64.00, z=83.84], EntitySheep['Sheep'/9, l='MpServer', x=-24.50, y=64.00, z=90.50], EntitySheep['Sheep'/10, l='MpServer', x=-23.50, y=64.00, z=92.50], EntitySheep['Sheep'/11, l='MpServer', x=-20.50, y=64.00, z=96.50], EntitySheep['Sheep'/15, l='MpServer', x=-2.50, y=64.00, z=63.50], EntitySheep['Sheep'/16, l='MpServer', x=-3.50, y=64.00, z=64.50], EntitySheep['Sheep'/18, l='MpServer', x=1.30, y=64.00, z=65.28], EntitySheep['Sheep'/19, l='MpServer', x=3.24, y=64.00, z=62.42], EntityCow['Cow'/20, l='MpServer', x=17.83, y=64.00, z=69.33], EntityCow['Cow'/21, l='MpServer', x=27.26, y=64.00, z=55.86], EntityCow['Cow'/22, l='MpServer', x=32.43, y=64.00, z=54.80], EntityCow['Cow'/23, l='MpServer', x=16.82, y=64.00, z=66.43], EntityCow['Cow'/24, l='MpServer', x=45.62, y=64.00, z=13.79], EntityCow['Cow'/25, l='MpServer', x=44.50, y=64.00, z=13.81], EntityCow['Cow'/26, l='MpServer', x=45.72, y=64.00, z=25.18], EntitySheep['Sheep'/27, l='MpServer', x=49.37, y=64.00, z=23.24], EntityPlayerSP['Dannythebrill13'/55, l='MpServer', x=3.52, y=69.97, z=38.75]]
Retry entities: 0 total; []
Server brand: fml,forge
Server type: Integrated singleplayer server
Stacktrace:
at net.minecraft.client.multiplayer.WorldClient.func_72914_a(WorldClient.java:532)
at net.minecraft.client.Minecraft.func_71396_d(Minecraft.java:2740)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:426)
at net.minecraft.client.main.Main.main(SourceFile:123)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

-- System Details --
Details:
Minecraft Version: 1.12.2
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 224806272 bytes (214 MB) / 637534208 bytes (608 MB) up to 3221225472 bytes (3072 MB)
JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx3G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP 9.42 Powered by Forge 14.23.5.2768 Optifine OptiFine_1.12.2_HD_U_E3 6 mods loaded, 6 mods active
States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored

State	ID	Version	Source	Signature
UCHIJAAAA	minecraft	1.12.2	minecraft.jar	None
UCHIJAAAA	mcp	9.42	minecraft.jar	None
UCHIJAAAA	FML	8.0.99.99	forge-1.12.2-14.23.5.2768.jar	e3c3d50c7c986df74c645c0ac54639741c90a557
UCHIJAAAA	forge	14.23.5.2768	forge-1.12.2-14.23.5.2768.jar	e3c3d50c7c986df74c645c0ac54639741c90a557
UCHIJAAAA	lunatriuscore	1.2.0.42	LunatriusCore-1.12.2-1.2.0.42-universal.jar	None
UCHIJAAAA	schematica	1.8.0.169	Schematica-1.12.2-1.8.0.169-universal.jar	None

Loaded coremods (and transformers):
GL info: ' Vendor: 'NVIDIA Corporation' Version: '4.6.0 NVIDIA 430.86' Renderer: 'GeForce GTX 1050 Ti/PCIe/SSE2'
Launched Version: 1.12.2-forge1.12.2-14.23.5.2768
LWJGL: 2.9.4
OpenGL: GeForce GTX 1050 Ti/PCIe/SSE2 GL version 4.6.0 NVIDIA 430.86, NVIDIA Corporation
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

Using VBOs: Yes
Is Modded: Definitely; Client brand changed to 'fml,forge'
Type: Client (map_client.txt)
Resource Packs: vanilla, file/Edited Default Textures (2).zip
Current Language: English (US)
Profiler Position: N/A (disabled)
CPU: 4x AMD Athlon(tm) X4 880K Quad Core Processor

Do you have Optifine and its "Render Regions" setting enabled? It seems to be an incompatibility, and once you disable it Schematica works fine again.

I too had this issue until I found the fix (Issue #321).