Regarding Jon W's comments, my husband uses open G tuning a lot, but also C tuning and a few others for different sounds. He plays in different keys while in any of those tunings, but sometimes uses a capo to move to a more familiar position for certain songs.

Banjo players also have been known to attempt retuning in mid song. I think there is a hearing impediment that develops from being so close to the banjo, which causes some players to retune often, whether it needs it or not. Or at least, I'm not hindered with such a fine ear that can notice the difference!