Gnomo, are you saying the members of the top philharmonic orchestras aren't expert musicians because they have the score in front of them?

I have an unusually good knack for memorzing things like script lines in a play or - especially - songs, and once I've learned a song, if I've sung it recently, I seldom need to have the music in front of me. That hardly makes me a great singer, musician, or performer. It certainly doesn't mean I'm better than someone else who needs to have the lyric sheet in front of them.

BTW, even when you know a song really, really well, it's quite possible to draw a blank, for just a moment, at the beginning of a line or verse.   When you do that in your living room or a singaround, the line will usually come back to you pretty quickly. But when you have that fleeting lapse on stage in front of a crowd, the pressure to resume the song without error gets in the way of recovery.   

That's what I find is a major function of a large-print lyric sheet.   Just knowing it's there, to be used for a quick prompt if needed, I hardly ever need to look at it.

To bring this to the topic, an actual legitimate excuse is just that: "I've sung this song a thousand times but my mind went blank for a moment."