I posted this yesterday but could not replicate it, but I found out how it happened. I figured out that if you head to either the End or the Nether, you can no longer hit escape to pause. It works again upon exiting to the Main Screen and then entering the SSP world again. EDIT: It also has to do with changing game modes from 1 to 0 or 0 to 1 in a LAN world, I am trying to figure out exactly how it occurs. EDIT 2: To recreate, create a new SURVIVAL Single Player world, open to LAN with cheats, change gamemode to 1, build a nether portal and go through it and you will no longer be able to pause with escape.