tac - concatenate or print in reverse (last line first).
tac > f1
1 2 Press CTRL+D to stop
tac f1
1 2
tac vid.avi > vid2.avi ; mv vid2.avi vid.avi
It wont play.
tac vid.avi > vid2.avi ; mv vid2.avi vid.avi
Back to playable.
NOTE: This is like a mini encryption for binary files.