Put the summary of the bug you're having here

What I expected to happen was...:
Nothing! The server I play on is perfectly fine, no lag, no nothing.

What actually happened was...:
After about playing for 15-20 minutes, Minecraft crashes. It's hard to explain how it happens though.

Steps to Reproduce:
1. Log into Minecraft as usual
2. Play on a server like normal
3. Minecraft unexpectedly crashes.
(Says in output log: A fatal error has been detected by the Java Runtime Environment. Also says: Failed to write core dump. Minidumps are not enabled by default on client versions of Windows.)