ALL THE TIME.
I've got a really weird kind of partial face-blindness, which means my brain tends to scramble known facial features, making it hard to remember exactly how it all fits together (though I have no issues recognising people in real life), so I forget stuff like this a LOT.
And while facial features are the biggest issue, I forget all sorts of things. I can't TELL you how many times I've accidentally drawn Masahiro holding stuff with his left hand. You know, the hand he doesn't have any more.
I almost always have the previous page open while I sketch the new one, just to make sure I'm staying somewhat coherent.