When my first baby was 3 months old, I left him on the bed for just a second and he rolled over for the very first time -- right off the bed. I was convinced he must have hit his head on the night stand on the way down, too, although I can't remember why I thought that. I FREAKED. I called my grandfather (who was a former football coach, and therefore knows quite a bit about head injuries). He proclaimed DS to be perfectly fine, but I still insisted on taking him to the pediatrician, who also proclaimed DS to be perfectly fine. He's 11 now, and a totally healthy, gifted child.
Another time, when my oldest DD was 5, she and I were napping on the couch together and... I don't know if I was having a nightmare, or I had a leg spasm, or what, but I seriously kicked her off the couch in my sleep! My leg just shot out and she went flying! I felt so horrible! She cried for a second, and then laughed because she couldn't believe that happened! She still laughs at me about that.