When we first started awhile back, we had those real cheap mats over carpet. The mats were super lightweight with "handles", kind of like handles on a purse or bag.
While we were doing ukemi, one of the students stepped on this handle and did a forward roll. His foot got caught up in the handle, and the mat wrapped him up like a burrito! It was so funny to see him rolled into that mat!!!
It was really funny when it happened. I still laugh when I remember that.