I was trying to kill some Zeds at the bottom of the stairs in Electro fire house (the one surrounded by the wire fence) and as I moved down the stairs, my fire axe appeared to be getting caught on the floor of the level above. This caused me to instantly side over the edge of the stairs when walking forward. This happened to me instantly twice out of two times, so I imagine it is readily reproducible. I was wearing a ballistic helmet and had a back-pack on which may or may not have had an influence.