Minecraft crashes when lagging

crash-2012-12-21_14.38.01-server
Occurred when killing a chicken while game lagging.

crash-2012-12-27_16.41.06-server
Occurred while using a map when flying (in creative),
game was severely lagging before crash occurred.

As you can see, my game is modded, however I created the mod.
These crashes are not caused by my mod to my knowledge.