I don't know if this is just because of the Optifine pre-release for 16.4, but I switched to the latest release yesterday and the / commands (i.e., /locate, /gamemode, etc.) do not work.  They appear as red text with the message "Unknown or incomplete command, see below for error at position 1: /<-[HERE]" above it. Again, I'm not sure if this is the version or the Optifine, but I figured it was important that I mention it in case it is an issue with the game.