Yep, seems a couple of things usually happen. I've had her disappear when out in the countryside, looked everywhere, couldn't find her, and finally found her on a rock. Unable to move from there, apparently.
The two things that happen most often - one, you go off and for some reason she can't take that tiny jump you took to get over a log or rock or whatever, and her pathfinding AI gets stuck and so she just stays in place. Often if it's just a matter of you went down a mountain in some way she can't follow, you can just hit the "wait" button and wait for a couple of hours and when you come out of wait she'll be there, having gone the long way around, etc.
The other thing that can happen is she sees an "enemy" of some kind - it can be a wolf, whatever - and she goes into attack stance and just stands there in that mode. The wolf just stays where it is, she stays where she is, meanwhile you have no idea she wasn't following you until you look back and she's gone. Often if you go back and find her in her attack stance you can simply talk to her and tell her "wait here" then "follow me" and get her out of that mode.
Also, I've had her "stuck" on a ledge and she just wouldn't take the little jump needed to get off, so a nice Shout of push would push her off the ledge, then off we go.
