Aiden Grimshaw was the true shocker exit of Series 7, but I'd say it was a sort of combination of consecutive bullshit decisions:
Treyc was shafted against Weasel's most abominable sing-off (Was the one where she pleaded "PLEASE believe in me!", LOL, the one vs. Treyc or vs. Belle Amie? Can't recall. Anyway....). Cheryl wanted to take it to deadlock, but the producers clearly manipulated the voting order as a means of saving Katie. They wanted her on tour to cash in on buying her out of her contract. Had it gone to deadlock, Treyc would've stayed as she had more votes.
Then Aiden was setup for the B2 by Simon the previous night. And as usual, favoritism was for the Weasel. They sent it to deadlock, and he was the real shocker of the season as most people expected him to be higher in the votes and clearly predicted him to go a little further. This is similar to Laura, Lucie, The Risk, etc., who were never really as high in the voting as people expected.
I'd say the main reason S7 broke the rule was due to it being offset by the Top 16 wildcards twist, which made matters more complicated.