For a Christmas party several years ago, my band learned Elvis' Blue Christmas. Now most songs we do start with an instrumental intro, so I have the key down pat before my vocal starts. This particular tune, however, starts with the "I'll have a Blue.." with the band coming in on the word blue. Every rehearsal was right on the money, but when we performed it in front of about 400 people, I started a full note higher than where the band came in. Instant panic ensued...you know, the kind where every body exchanges a panicked glance that translates "Oh SHIT! Now what do we do?!!" With a loud "Merry Christmas, Every one!" I ended the song, and we went right into the next song on the list.