Of course if you're accompanying yourself on an instument you could play until you remember the words - or schithereee idle doodle your way through it. I think politeness usually allows a person time to 'recover'and carry on especially for those who may be nervous and/or not used to 'performing' in front of an audience, like me,(especially if your likely to get 'gong-ed' or penalised). Usually the person acting as mc will let you know if its one or two songs in your slot. If i fluff on the first song, i would embarrasingly go on to the second. If i fluff on that aswell, i'd apologise and sit down. I've found people are usually quite understanding and sympathetic - sort of 'don't worry mate, we've all bin there' etc.