I was simply travelling by boat in an open ocean (biome: Deep Ocean) while having my map of the region open in front of me. The framerate then got incredibly bad (debug was showing 0 FPS before the crash) and then the game crashed on its own, with the launcher showing the crash report (see attached).

The basic crash information is as follows:

Time: 12/22/13 12:00 AM
Description: Unexpected error

java.lang.IllegalStateException: Already tesselating!
at bna.a(SourceFile:159)
at bna.b(SourceFile:154)
at bmt.a(SourceFile:892)
at bmm.a(SourceFile:941)
at bmm.b(SourceFile:814)
at azi.ah(SourceFile:787)
at azi.f(SourceFile:711)
at net.minecraft.client.main.Main.main(SourceFile:152)

For a more detailed explanation, please see the attached file.