I ask people to come up and play all the time. We have a pretty tight little musical community up here, and bass players always get extra love from yours truly.
We did have a situation a while back, with some friends of ours from another band. We ask them up all the time, but on this particular occasion, the bass player was babysitting the guitar player who was going through a nasty divorce. The guitar player was drunk as a skunk and getting drunker with each passing minute. On about three songs, he just came out of nowhere, grabbed the closest mic, and proceeded to sing background vocals, or even the lead on one tune in particular. This wouldn't be a big deal under normal circumstances, but he was too hammered to carry a tune. It was really surreal. It was annoying and yet strangely entertaining in a train wreck sort of way. The bass player was very apologetic, and I even asked him to play a song or two. (He declined as he had to watch the guitar guy to make sure he didn't aspirate any vomit or anything.)