This has happened to me a few times, with different bugs. Usually this happens to us modders, (especially NMM users) because we are constantly adding, deleting, changing, doing whatever to our game, which ends up in dead-end in scripts, and other 'bloated' problems within our games. This happned one other time, during Bleek Falls Barrow, which was a main quest. You would go up to it and nothing would happen.
Word wall stopped working for me in Ustrangrav, the 'turn etheral' word wall.
