I leaned down to the right and reached for a brush, the Garand tilted toward the barrel end and the follower touched my thumb, the bolt slammed forward and I had the whole thing hanging off the end of my thumb.
Sounds like the bolt wasn't locked back, but just hung up on the follower. I'll bet it looked something like Chris_B's photo.
I've never had a M1 close while my thumb was pressing on the top round in the clip.
I do make sure the bolt is locked all the way back before sticking my fingers in there to clean the chamber or anything before firing.