Single Player Commands compatibility since it wasn't going to be updated beyond 1.6.2. Are you using some unofficial SPC for 1.6.4? If so, where is the Minecraft forum thread or (legit) website for it? Does it require Forge and/or Liteloader now, and if so which version?
Hah. i almost forgot that there is no 1.6.4 official version, but i cannot remember how i got ahold of this, i checked the mods folder it says its "SPC-1.6.4" and in game when i do /SPC version it says 5.2.2.
EDIT: i did some sniffing around and i found this (http://www.6minecraft.net/single-player-commands-mod/) although i can't confirm that is where i got it from, but it might be.