I had a similar experience last year when we booked Dave Swarbrick. Understandably, the evening was a sellout and there were a number of people there with instruments who obviously had it in mind to play if there was a spot, but I announced early on that on this occasion, unusually, I had arranged floor spots in advance as I wanted to present the highest standard that I could and on this occasion there would be no newcomers to the club.
The next day I received a MySpace request for friendship to the club's MySpace from a duo and when I looked at their site, I saw that they had advertised that fact that they were appearing at the Royal Oak in Lewes on the night that we had booked Swarb! They were not even claiming that they were playing a support spot.
I emailed them to asked about the honesty of this claim and they replied saying that they had turned up prepared to do a floor spot so they thought this was OK.
However they had not:-
* contacted me in advance saying that they wanted to play
* approached me on the evening for a floor spot
* asked whether they could advertise themselves as appearing at our club