My Fiance and I are experiencing crashes at random times while playing.

[11:19:50] [main/ERROR]: Exception loading model for variant tinker_io:upg_8#inventory, normal location exception:
net.minecraftforge.client.model.ModelLoaderRegistry$LoaderException: Exception loading model tinker_io:item/upg_8 with loader VanillaLoader.INSTANCE, skipping
at net.minecraftforge.client.model.ModelLoaderRegistry.getModel(ModelLoaderRegistry.java:153) ~[ModelLoaderRegistry.class:?]
at net.minecraftforge.client.model.ModelLoader.func_177590_d(ModelLoader.java:314) ~[ModelLoader.class:?]
at net.minecraft.client.renderer.block.model.ModelBakery.func_177577_b(ModelBakery.java:175) ~[cgb.class:?]
at net.minecraftforge.client.model.ModelLoader.func_177570_a(ModelLoader.java:164) ~[ModelLoader.class:?]
at net.minecraft.client.renderer.block.model.ModelManager.func_110549_a(ModelManager.java:28) [cgc.class:?]
at net.minecraft.client.resources.SimpleReloadableResourceManager.func_110542_a(SimpleReloadableResourceManager.java:121) [cev.class:?]
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:512) [bib.class:?]
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:377) [bib.class:?]
at net.minecraft.client.main.Main.main(SourceFile:123) [Main.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_25]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_25]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_25]
at java.lang.reflect.Method.invoke(Method.java:483) ~[?:1.8.0_25]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
Caused by: java.io.FileNotFoundException: tinker_io:models/item/upg_8.json
at net.minecraft.client.resources.FallbackResourceManager.func_110536_a(FallbackResourceManager.java:69) ~[cei.class:?]
at net.minecraft.client.resources.SimpleReloadableResourceManager.func_110536_a(SimpleReloadableResourceManager.java:65) ~[cev.class:?]
at net.minecraft.client.renderer.block.model.ModelBakery.func_177594_c(ModelBakery.java:334) ~[cgb.class:?]
at net.minecraftforge.client.model.ModelLoader.access$1600(ModelLoader.java:130) ~[ModelLoader.class:?]
at net.minecraftforge.client.model.ModelLoader$VanillaLoader.loadModel(ModelLoader.java:880) ~[ModelLoader$VanillaLoader.class:?]
at net.minecraftforge.client.model.ModelLoaderRegistry.getModel(ModelLoaderRegistry.java:149) ~[ModelLoaderRegistry.class:?]
... 14 more
[11:19:50] [main/ERROR]: Exception loading model for variant tinker_io:upg_8#inventory, blockstate location exception:
net.minecraftforge.client.model.ModelLoaderRegistry$LoaderException: Exception loading model tinker_io:upg_8#inventory with loader VariantLoader.INSTANCE, skipping
at net.minecraftforge.client.model.ModelLoaderRegistry.getModel(ModelLoaderRegistry.java:153) ~[ModelLoaderRegistry.class:?]
at net.minecraftforge.client.model.ModelLoader.func_177590_d(ModelLoader.java:322) ~[ModelLoader.class:?]
at net.minecraft.client.renderer.block.model.ModelBakery.func_177577_b(ModelBakery.java:175) ~[cgb.class:?]
at net.minecraftforge.client.model.ModelLoader.func_177570_a(ModelLoader.java:164) ~[ModelLoader.class:?]
at net.minecraft.client.renderer.block.model.ModelManager.func_110549_a(ModelManager.java:28) [cgc.class:?]
at net.minecraft.client.resources.SimpleReloadableResourceManager.func_110542_a(SimpleReloadableResourceManager.java:121) [cev.class:?]
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:512) [bib.class:?]
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:377) [bib.class:?]
at net.minecraft.client.main.Main.main(SourceFile:123) [Main.class:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_25]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_25]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_25]
at java.lang.reflect.Method.invoke(Method.java:483) ~[?:1.8.0_25]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
Caused by: net.minecraft.client.renderer.block.model.ModelBlockDefinition$MissingVariantException
at net.minecraft.client.renderer.block.model.ModelBlockDefinition.func_188004_c(ModelBlockDefinition.java:83) ~[bvv.class:?]
at net.minecraftforge.client.model.ModelLoader$VariantLoader.loadModel(ModelLoader.java:1189) ~[ModelLoader$VariantLoader.class:?]
at net.minecraftforge.client.model.ModelLoaderRegistry.getModel(ModelLoaderRegistry.java:149) ~[ModelLoaderRegistry.class:?]
... 14 more

We're both using 2.6.1 Tinker I/O
And we've tried Forge 14.23.3.2657 and an older version (don't remember which, Twitch Client did it for us at first).

My Fiance has been crashing a lot more than I and we'd been trying to figure out why since our setup is indentical mod-wise as we're playing on LAN and I've only experienced this crash error once before. :(

I will rewrite block and item register to fix the problem. Thanks for your report!

The problem was solved in the Tinker I/O rewrite version! Thanks for the report!