Another thread (the floorspots one) mentions someone's unfortunate timing for a song.
I'm sure that this must be a rich comedic seam.
The two that spring to mind for me are:
Singing The Parting Glass at a barbecue where the host had just split up with his long-term partner (oops).
Singing Rise Again in the front bar of the pub, whilst a funeral party was being held in the back room (d'oh).

Its usually halfway through the song that you realise.....

These two are pretty unspectacular - but there have to be some classics out there. Yes?

Kris