¥Someone's overpowering perfume triggers an athsma attack
¥You really have to belch but can't quite get it out
¥A lady with stage 3 Alzheimer's tries to take your guitar away from you mid-song
¥A drunk in the audience is 'harmonizing' with you at the top of his lungs but doesn't know the words

¥You've just screwed up the words ("...I heard your face and saw you call my name...") and everyone else is doubled up laughing, and now you are, too.  (This happened to me last Monday.)