I dont know how true this is but I saw it around a few places so decided to post it up here. The fan account goes like this: the girls were recording for Inkigayo and Yoona’s hair hit her in the eye and Seobaby was so shocked/surprised she cried. It wasn’t too serious but the pain shook Yoona a bit and she stopped dancing. Recording was paused for a bit.
By hair? Maybe the account was lost in translation but I’ll definitely need to find some more confirmations. Here’s a picture though, credits to dorkiiilove. She THINKS this is the picture of Yoona after the accident. Looks like it though since the girls are surrounding her.