Often when i'm playing with people and someone else is leading. Seldom when I'm leading. Lots of times I'm playing with people I've never met before, or maybe once before, and they call something I'm familiar with to a degree, but have never played it before. As long as someone else can do the melody, that's usually enough to get my mind working to remember the changes. Especially if the bass player is good and can lead into the changes as they come. But yeah, I'm often playing tunes I barely know or haven't thought about in ages. It's a little tense on the nerves, but really good for me as a player. Keeps my ear working; and also reminds me of some great tunes that I should know better.