It's like the bit in the cartoons where the character walks straight out off the building, with no problem - but then looks down, realises what's happening, and starts to fall.

You never know a song so well it won't happen some time. Basically there are two ways to go - use tricks to cover up, or be open about it. Either can work. But make your mind up which you are doing.

I remember a concert where Eric Bogle blanked completely in a song he knew backwards. He stopped - said something like "This song deserves to be sung with me knowing the words - I'll get back to it" and then he sang another song, and did get back to it afterwards; no problem.

With a song you've got the tune, so you can sing nonsense syllables or play an instrumental break or whatever. When I blanked in a spoken monologue a few weeks back I couldn't see any way out but stop. I suppose I could have switched into the approach adopted by the Kippers with their wordless whistled monologue, but I didn't think of that.