Minecraft 14w19a, Minecraft 14w20a, Minecraft 14w20b
See Hopper crash report with ID 68245197
By setting the GameType field of a level.dat file to 3, the game crashes when clicking the Singleplayer button from the main menu. Values of 0, 1, and 2 work fine as expected.
The crash is an ArrayIndexOutOfBoundsException, most likely when trying to get the text to display for either survival, creative, or adventure mode in the world list.
Fixed in 14w21b