I searched for this bug report but could not find it here, only on the boards.

If you're carrying your primary firearm in your hand and going up one of the following stairs:

• scaffolding stairs at castle towers
• metal stairs of large red, metal barn building (the one with scaffolding on top floor and a flight of stairs on the side)

... and switch to a secondary firearm (in my latest case the MP5) ...
.. the character teleports to the side by roughly a half a meter which can cause you to fall down and break your legs or die.

I've gotten myself into quite a fix on a number of occasions because of this glitch. I think it even happened once while I was reloading