I built a mob spawner and it was working fine. A few hours later, no mobs were spawning. The entity count was at 33. I logged out, and back in and it worked for another 2 or so hours. My mob farm was created above a small lake high above a mushroom biome(the whole farm was in the ocean biome).
While having this issue: Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (
[minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/finding-minecraft-data-folder]
) to this ticket.