I stumbled across this a couple weeks ago while doing the main quest.
In Sancre Tor, the Reman Vault there's a draw-bridge that's suppose to be lowered with a handle I believe. When I arrived in the area the bridge was already lowered but I couldn't walk across it, I would fall through the bridge instead. I tried several times to talk across the bridge thinking I'd hit an "odd spot" but fell through every time before it occurred to me that this was an error. I pulled the handle to lower the bridge and "heard" the bridge lower. At which point I could walk across the bridge without and issue.