When starting as the Attackers on Gallery, the area on the left (the boarded up side, on the corner of going to bombsite A) can be jumped over:
Jump on the bench on the left of the spawn, and then double-jump over the boarded up area.
Seems to be abused quite a lot at the moment (though it’s pretty useless, as you can’t see/shoot the enemies inside the boundary of the map).
Edit: Looks like the ‘Prefix’-drop-down menu (to categorise the bug) does not contain the 3 Execution maps.
Edit2: Added screenshot: http://images.akamai.steamusercontent.com/ugc/1617176672232155295/D242F7E61CDCFE06F41E5148EDE70F84A2DB1B6F/