Ali's Pretty Little Scar/ Twin Theory on PLL
I think Ali's scar could be from the night that Ali crashed Spencer and Emily's sleepover to avoid Jason and his party guests. She seemed very shaken up that night, we saw her crying, and this was never explained. Ali had a similar tone in tonight's episode when Hanna asked her about the scar.
If I am right about that scar originating from the night that Ali crashed Spencer and Emily's sleepover, then that night would also be an interesting time for a twin switch IF the show did decide to take the twin storyline from the books. *I know that the writers have said that Ali does not have a twin, but they obviously wouldn't tell us if she did.* It's semi out of character for Alison to have left a party like that, upset and seeming afraid. The Ali we know hung out with an older crowd all the time and had no problems that.
Also, before Ali arrived, Spencer and Emily were discussing Jason's parties and Ali as if they were not yet that close with Ali. In the books, Ali's twin befriended the girls because she had to quickly make people believe that she was Ali, and to do that she needed a new group of friends, because she wouldn't have known enough about Ali's old clique. I'm also pretty sure that Marlene King has stated that we find out in this season why Ali "chose the girls" as her friends, so that would make sense. This could be why Ali was so freaked out that Hanna and the doctor found the scar; perhaps it can prove that she isn't who she says she is.
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Thanks for reading, and be sure to let me know what you think!