Some of the WoW UI code uses MAX_PLAYER_LEVEL_TABLE[GetAccountExpansionLevel()] to determine the current player's level cap—I know this code is used in FrameXML/ReputationFrame.lua, and I used it in Pawn, and I'm sure it's used in a bunch of places also. Now that the Mists of Pandaria preorders are available, GetAccountExpansionLevel returns 4, but MAX_PLAYER_LEVEL_TABLE doesn't have a [4] if your running on WoW 4.3 realms, so all of that code breaks now for anyone who's upgraded.
Blizzard—for backward compatibility (including with Blizzard code in the current live client) maybe the server should never send anything higher than 3 for whatever GetAccountExpansionLevel uses on live realms. (Send the actual value on the PTRs and beta realms of course.)
AddOn authors—Time to write a special case!
