As a singer...I dread the thought of forgetting the words to a song....and it DOES happen from time to time. It's not quite so bad singing with a choir....although in some pieces (Palestrina, Gabrieli, Victoria, Byrd etc. ,etc) you can have up to 11 completely different and totally independent ployphonic 'parts' weaving around you and so one has to be alert! Sometimes this 'blanking' phenomenon can be experienced en masse. At one concert last year the entire first tenor section (including myself) completely failed to come in at a particular entry during a piece that we'd all sung many times before (and never gone wrong before). Just one of those things - perhaps I should add that our choir is not allowed - by the conductor- to perform with music so everything has to be committed to memory.
It can happen to anyone. A few years ago (quite a few years!) our choir had Edmund (Ted) Hockridge (who sadly passed away recently at the grand old age of 89) as our guest artist. Now, he was recording artist of some repute as well as having headlined many West End shows, toured as a solo artist etc., etc. Following an 'audition' of the choir ...he decided that we were good enough to perform choral backing for him on several numbers. It was very enjoyable and he was a great showman with a tremendously powerful voice. At that time we had amongst our ranks a fantastic alto singer with a voice reminiscent of Kathleen Ferrier - why she never turned fully professional I'll never know. But anyway....she was so good (she was a music teacher and vocal trainer as well as a superb soloist) that Ted Hockridge wanted to do a couple of duets with her. We all thought that their performance of "No Two People" was wonderful - especially the parts where they appeared to coyly whisper and almost 'spoon' during the instrumental interludes. So, we were puzzled by her raging about 'That man, so unprofessional!' during the interval. It transpired that Ted had just gone completely blank re: the lyrics of the song. The 'coy whispering' was, in fact, - on every verse - Ted desperately asking her what his next set of words was. From the way he sang and performed though, the audience would never have guessed.
Rob