Online purchases can be so iffy. Some items just can't be trusted and for big events like the senior prom you are probably better off just spending the extra money in a store so you can see the quality.
I do remember a girl in high school who lost her sundress as she slipped into class late. It caught on the door which she tried to close quietly so the teacher wouldn't notice. She took one step and the dress was stuck in the door, the simple thin strap that tied behind her neck untied instantly and there she was standing in her underwear, no bra. I was one of 3 people that noticed as she managed to keep quiet and back towards the door pulling her dress up. The other two were girls and friends of hers so no big deal.
I had quickly stood up to help her open the door back up and she ran off to the bathroom. at this point everyone had turned around confused so I told them she thought she was going to puke so she was running to the bathroom. The girls that saw what happened asked to go check on her and told her nobody knew what happened. Looking back it was pretty funny, but at the time she was mortified.