Owie! I did that misstake when i did my first backroll. I got head stuck between mat and shoulders. I tried rolling straight backwards instead of over the shoulder. Damn that hurt.
I have never had that accident, but I had my share of Ukemi accidents. Once Sensei got me so worn out (oops...doesn't sound that good the way I put it sorry) that at one of the very last front rolls I lost all my form and pancaked on my shoulder with the body folding onto it (and I weight 230 Lb.). Result, bruised ribs that hurt for several weeks.
As backrol I can remember only one incident. I don't recall the technique, but I was falling backwards with Sensei behind me pulling me down. I took Ukemi and my head went streight onto his knee. That made me realize that I actually can't control yet Ukemi, but I fall as a programmed robot...same place same procedure. In fact the usual Ukemi didn't work in that case and my head bent forward more than it usually does...big pain.