Serana has been an adept follower for me, but this evening she got lost in Gloomreach. She made it as far as the "Hive" but then stalled on the ledge above the Falmer village. Most disturbing, she did not teleport to my location when I transitioned back into Gloomreach proper, nor when I exited to Skyrim. I went back to where she was and tried setting her to wait and then follow. She remained stuck on the ledge. The only way to extract her was to exit the way I came in. This was not a ledge requiring a jump-down, by the way. Apparently she didn't have the ramp on her map.