Image available @ http://puu.sh/6Eslh.jpg
Place end portal blocks on the ground somewhere, take over a mob (i'm in a cow in that picture), wait patiently for the mob to hit the portal.
When the mob touches the portal, you freeze, you can't move, you can't use shift to get out, if you change your game mode it makes no difference, basically you are stuck.
Fixed in 14w25b. Now when a spectated mob touches a portal, you no longer spectate it and the mob goes through it.