I was just walking around fort dawnguard (inside), and the floor in the lobby disappeared, thus causing me to fall through the game world. I exited and then re-entered the fort, which seemed to fix it, but I can see this becoming annoying if it happens too often

had that happen as well

I've had it happen in like.. Whiterun. I'm pretty sure this one came before Dawnguard.

YES! This happened to me in Whiterun as well! All the guards were falling in the hole with me, talking about whether or not I was dragonborn. Were you playing a new character or one you had created before the DLC! This had not happened to me before I downloaded the DLC. Coincidence? Or did the DLC trigger something? [Edited to add content]