The Promise - S6-E13
Stupidity: Markham lets Ava call her uncle's phone, and she doesn't let on that Boyd answers instead. He later says "I don't trust anyone", but he didn't demand she put the call on speaker for transparency, he's quite happy (at the time) to just hear one side of the call.
The Hammer - S1-E10
Character mistake: Raylan Givens tells Judge Reardon that he carries a ".45 Glock" but this is untrue. Throughout the series with the exception of the beginning of the first episode Raylan Givens carries a Glock 17, which is only chambered in 9mm parabellum. His backup can be seen to be a Glock 26 which is also in 9mm.
Chief Deputy Art Mullen: Just what part of "under investigation" confuses you?
Raylan Givens: So many things confuse me, Art.
Whistle Past the Graveyard - S5-E8
Question: Boyd and crew nearly ruin their relationship with Yoon by killing the Memphis crew in Mexico, as they were warned not to kill anyone south of the border. They redeem themselves by saying they'll take care of the bodies themselves and get them into the US. All well and good until the Mexico police stop them, and the crew let them take the truck with the bodies, congratulating themselves on the deception...but how does that solve anything? Corrupt or not, the cops now have the bodies, in Mexico.