I heard that when Southwest first took delivery of the -700 series (their first experience with vac toilets) one of the standard FA gags was to unroll a roll of toilet paper down the aisle, then stick an end if it in the toilet. Upon flushing the toilet paper would scream past seated passengers at eye level at about mach 3.8 and vanish in a second or two.

The story goes that maintenance put the kabash on that gag after they grew quickly tired of servicing broken lavs.