Unfortunately, sometimes, nobody seems to tell the house bands. An open mike I used to go to the house band, and sometimes any old band, would get up and play 6 or 7 songs in a row, and even then individual members would just stay on stage all night. Makes it very hard for someone who is a bit less confident to get up and do their thing.
Edited to add, one of the problems was that the organisers, who we're both nice people, would go out for fags and who ever was on at the time would carry on until one came back in.