I get asked to play songs we've never rehearsed (and in some cases, songs I've never heard), all the time. But it's the nature of the band I'm in.I had a BL mention that we were going to do a song at a gig that we had never rehearsed but "everybody knows". I said, politely, that I refuse to play something that we have not rehearsed, unless it is a request accompanied by a $20 bill. He hasn't asked since.
About 2/3 of our gigs are country dances, with an older crowd, in community halls that seat up to 1000 people.
Our OnSong has a list of about 1500 songs, and occasionally our BL will pull up some obscure old time waltz from the late 50s and I just read the chart. He'll say "it's got an early Loretta Lynn feel to it" and off we go.
I love the challenge. Keeps the night interesting!