Crash with Gliby's Voice Chat when leaving a singleplayer world and creating/joining a new singleplayer world, I haven't tried in multiplayer. This only happens with LWJGL3ify installed, this happens consistently, all you need to do is join or make a world, leave the world and then make or join a new world, it will crash.

[19:29:07] [Server thread/ERROR] [FML/]: Caught exception from gvc
java.lang.UnsupportedOperationException
at java.base/java.lang.Thread.stop(Thread.java:1654) ~[?:?]
at RFB-Launch//net.gliby.voicechat.common.VoiceChatServer.stop(VoiceChatServer.java:198) ~[VoiceChatServer.class:?]
at RFB-Launch//net.gliby.voicechat.VoiceChat.stopServer(VoiceChat.java:120) ~[VoiceChat.class:?]
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
at RFB-Launch//cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:532) ~[forge-1.7.10-10.13.4.1614-1.7.10-universal.jar:?]
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
at RFB-System//com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.dispatch(EventBus.java:322) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.post(EventBus.java:275) ~[guava-17.0.jar:?]
at RFB-Launch//cpw.mods.fml.common.LoadController.sendEventToModContainer(LoadController.java:212) ~[LoadController.class:?]
at RFB-Launch//cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:190) ~[LoadController.class:?]
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
at RFB-System//com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.dispatch(EventBus.java:322) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.post(EventBus.java:275) ~[guava-17.0.jar:?]
at RFB-Launch//cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:119) ~[LoadController.class:?]
at RFB-Launch//cpw.mods.fml.common.Loader.serverStopped(Loader.java:850) ~[Loader.class:?]
at RFB-Launch//cpw.mods.fml.common.FMLCommonHandler.handleServerStopped(FMLCommonHandler.java:483) ~[FMLCommonHandler.class:?]
at RFB-Launch//net.minecraft.server.MinecraftServer.run(MinecraftServer.java:489) [MinecraftServer.class:?]
at RFB-Launch//net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685) [?:?]
[19:29:07] [Server thread/ERROR] [FML/]: A fatal exception occurred during the server about to start event
java.lang.UnsupportedOperationException
at java.base/java.lang.Thread.stop(Thread.java:1654) ~[?:?]
at RFB-Launch//net.gliby.voicechat.common.VoiceChatServer.stop(VoiceChatServer.java:198) ~[VoiceChatServer.class:?]
at RFB-Launch//net.gliby.voicechat.VoiceChat.stopServer(VoiceChat.java:120) ~[VoiceChat.class:?]
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
at RFB-Launch//cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:532) ~[forge-1.7.10-10.13.4.1614-1.7.10-universal.jar:?]
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
at RFB-System//com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.dispatch(EventBus.java:322) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.post(EventBus.java:275) ~[guava-17.0.jar:?]
at RFB-Launch//cpw.mods.fml.common.LoadController.sendEventToModContainer(LoadController.java:212) ~[LoadController.class:?]
at RFB-Launch//cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:190) ~[LoadController.class:?]
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[?:?]
at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[?:?]
at RFB-System//com.google.common.eventbus.EventSubscriber.handleEvent(EventSubscriber.java:74) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.SynchronizedEventSubscriber.handleEvent(SynchronizedEventSubscriber.java:47) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.dispatch(EventBus.java:322) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:304) ~[guava-17.0.jar:?]
at RFB-System//com.google.common.eventbus.EventBus.post(EventBus.java:275) ~[guava-17.0.jar:?]
at RFB-Launch//cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:119) ~[LoadController.class:?]
at RFB-Launch//cpw.mods.fml.common.Loader.serverStopped(Loader.java:850) ~[Loader.class:?]
at RFB-Launch//cpw.mods.fml.common.FMLCommonHandler.handleServerStopped(FMLCommonHandler.java:483) ~[FMLCommonHandler.class:?]
at RFB-Launch//net.minecraft.server.MinecraftServer.run(MinecraftServer.java:489) [MinecraftServer.class:?]
at RFB-Launch//net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685) [?:?]

I don't know what's causing it, it may be GVC or it may be the reason GVC crashes but the singleplayer server doesn't get shut down when leaving, this is a heapdump taken on the main menu after leaving a world: