The title pretty much says it all. Making a world with the Caves of Chaos preset causes the terrain to go to 256 rather than 255, where there are large flat areas. This also causes a strange quirk where hostile mobs will spawn on those flat areas regardless of the time of day, and undead mobs don't burn during the day. This is probably because the light level doesn't exist at that height, according to the debug screen. I'm not sure if another issue should be opened for this spawning, as blocks aren't supposed to go to 256 anyway. (I was originally going to report a bug for the mobs spawning during the day, but then I realized it was because the terrain was higher than it should be.)

(And forgive me if I'm doing this wrong. This is my first time reporting a bug.)