Well, mine are 9, 7, and 1, and I wouldn't know which "bad mommy" story to tell you first! Not that I really have THAT many, but things happen, even if you're ultra-careful. Okay, here's one of my favorites...
We had just moved Buzz (then 2) up to his "big boy bed", which of course had rails on it. We tucked him in, and left the room. Well, I'm next door in Belle's room, when I hear Thud! Waaah! So, I know what happened, I race down the hall, throw open the door...and promptly whack him in the head with it! ARRGH! He was fine after he calmed down, and I can chuckle over it now, but I felt absolutely awful.
She survived, you'll survive and be more careful next time. Just wait until she's older and you have such fun things as jumping down the stairs and breaking an arm (Belle), doing a cartwheel and breaking an arm (Belle again, same arm), falling off a playset (both kids), falling into the pool...