When I was taking Japanese jujitsu, we were practicing throws and rolls and I did something to an abdominal muscle. I felt a sort of "scrinch!" and it hurt then, but it was the next morning when I *really* felt it! It was tortuous. I missed a day of work because I simply couldn't get out of bed, or up and down from a chair without my husband pulling me up! It was awful.
Needless to say, I was much, much more careful in class when we worked on throws and rolls. I still got sore occasionally, but I didn't strain another ab muscle, thank goodness! That was a bad couple of days.