Here's my ugly list of "silly mistakes", those miserable "I-knew-it-but-I-marked-something-else" bastards. I've committed all of them and that's why they're here.
"My hand itches" syndrome
Not reading the question completely. Your hands are so itchy to scribble down that equation even before reading the friggin' question! READ the damn question first, and then write your equations. "Jim travelled 90 miles less than Sally did in her first leg of journey" is NOT J = S-90. There's probably something more than that!
"I know more than the test creators" syndrome
Making your own assumptions based on your knowledge. Yes, maybe the beauty queen from your country won the bikini contest in 2001, but if the paragraph says "The potbellied 3 ft midget from Angorra won the contest in 2001", then that is what you should consider.
"I do everything really fast" syndrome
No one is giving you marks for "doing your questions fast." Just like it doesn't help to do a sprint in bed (if you're a virgin that's a tip for you), stop being an idiot trying to do 15 DS questions in 4 minutes. Pace yourself well. Read the choices really carefully.
"I will conveniently ignore the signs" syndrome
Even If ab <> , it does not mean b <>