I once had a gig where the local morris side, who were hosting the ceilidh, insisted on practising their set during the sound check.

That was just one of a series of disasters that evening involving filthy weather and broken down vehicles resulting in missing PA (and potentially missing musicians, although eventually they all turned up), added to the tension caused by one band member having to phone home frequently to check that his wife hadn't gone into labour. Too long a story to relate in full here, but cumulatively it all addeed up to my worst gig ever - although the ceilidh itself went fine.