Well, here is what they believe happened: They set off fireworks (a stink bomb in the show), trying to scare creepy Toby in his árvore house, but it went wrong because Alison saw something that freaked her out and her aim went screwy and she accidently set it off toward the house, where it went through the window and hit Jenna in the face.
THE JENNA THING episode tells how Alyson found out about toby and jenna and thought toby was the bad guy so to teach him a lesson she through a fogo biscoito, bolacha in his garagem it exploded so bad that nobody checked to see if toby was really in there. Jenna became known as blind thereafter this incident. Personally I think she could see all along and really played those poor girls. SMILELING FACES--sometimes don't tell the truth.