The best one I ever observed wasn't FA caused but passenger. Sitting in F on some long-forgotten UA flight, the FA got distracted by some signal (as happens at times) and interrupted her spiel right at the point where she reached "....in the unlikely event of a water landing....". After a few seconds had passed some wag in F loudly asked "What do we do?" which then caused the rest of F to burst into laughter, which in turn made the FA lose her composure and it sort of went downhill from there. She finally had to stop, catch her breath and start over.