Oh God! I have thousands of these
The standard father of the bride who gets up to do "a few bars" of invariably Delilah, 'sweet caroline' or 'the candy store on the corner' in his own unique rhythm and pitch (I swear to god one guy went from 5/4 to 3/4 up to 7/4 one night and the song changed key about 20 times in the first verse alone).
The guy who asks to get up to sing 'Elvis' who disappears for a few minutes only to re-appear in full 'Fat Elvis the vegas years' regalia..wig, sideburns, shiny belt and all...I swear I nearly puked I was laughing so much.
Riot between the two families In THE MIDDLE OF THE FIRST SONG

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The electricity grid going down in the area just as we were about to start....by the time it came back on (3 hours later) we could only play one song before the premises were shut (in fairness they still paid us though...easiest money I ever made)
Drunken singer who in fairness hid it brilliantly...never missed a cue or a line and sang in pitch all night UNTIL he announced the next song was by 'Simon and Carabunkle', sang 'Teddy bear' as 'Pubic hair' and started Chris De Burghs lonely skies as 'My underpants they call Labeeze are swirling down around my knees......'.
And the best.....The gang of 'hard cases' who decided to gatecrash a wedding not realising it was a 'special forces' bash....needless to say they got a good lesson in manners....most entertaining wedding I was ever at.