Barney breaks the fourth wall when before he tells Betty what his practical joke on Fred is going to be. He tells the audience "Pardon me, but this is such a gasser, I can't take a chance of Fred finding out about it." He then proceeds to whisper the joke to Betty. Betty is also looking at the audience part of the time Barney is talking to them.
ABC Originally Broadcast This Episode In Black And White
When Fred Knocks on Barney's basement door, Barney tells him to "lock up after you're in," yet when Fred enters, Barney is the one that locks the door.