Well, there's misjudging the existence and validity of Pinkie Sense, arguing that it wasn't Gilda's problem that she didn't like Pinkie, eventually going along with her friends believing that Zecora is an "evil enchantress", believing that she needed to send a friendship report to Princess Celestia exactly every week, misidentifying the Equestria Games inspector along with her friends, too easily dismissing Fluttershy's solution to the bat problem along with her friends (again), and believing that Rainbow Dash could still study for the Wonderbolts exam using flash cards despite evidence to the contrary.  There's probably more though.

If I ever needed proof Twi isn't a Mary Sue, this would be it. Many thanks fairweather friend.