I was experimenting with the new spectator mode (/gamemode sp).
What I expected to happen was:
Hitting the item frame would not do anything.
What happened was:
I froze, not being able to do anything and standing on the top of the block on which the item frame was placed. The F3 debug screen indicated my view direction was changing (east, north, west etc), but I was still looking in the same direction. To unfreeze, use the shift key (anything else diesn't work) or change your gamemode.
What happened the first time: I did unfreeze again, and I used the spectator mode as normal, but could not pass through the ground and collided with blocks. I furthermore recieved an 'attempting to hit an invalid entity' error message.
Steps to reproduce:
Place an item frame on a block
Put yourself into spectator mode
Attempt to hit the item frame
To unfreeze: Use shift (descend) or change gamemode
Same here.