It happened to me: one moment I was sitting at the table in my IKEA chair, perhaps leaning forward to browse HN, and the next I was on the floor with (the mirror revealed) a deep 20-inch gash across my upper back, and a lighter slash behind my knees/legs.
Obviously the chair broke, but I still haven't figured out how it cut me so severely. A few inches higher and it would have been my neck.