This was awful.
I was getting ready Emma for school, and as I took off her PJ shirt, I noticed something really weird about her arm. At first, I couldn't tell what it was-- even after staring at it for a few seconds. It looked so odd that nothing was registering.
There was a really deep crease circling around her upper arm. I was afraid it was a cut... but there was no blood? A quick prod revealed that she had managed to get one of her elastic rubber hair tyes onto her upper arm at some point during the night (presumably while she was still awake in her crib before falling asleep), and it was cutting off the circulation in her arm!
I removed it immediately, completely freaked out. It hadn't seemed to be bothering her before I removed it, but I think the immediate circulation of blood must have really hurt, because she started screaming right away. She cried, and whimpered, and held her arm all day. They even had to call me to pick her up from school!
Bless her heart, I felt so awful. As you can see in the picture, we ended up strapping a cold pack to her arm, which seemed to help with the pain. She's had a mark on her arm for a while now, with no signs of it disappearing!
So word to the wise: make sure all the elastics are out of little girls' hair before bed time!