Duck/Aren't You Chupacabra to See Me? - S4-E9
Continuity mistake: Mandy is in the library working on her report. When she's working on the second page, the writing is scribble, and on top of a stack of paper. A second later, the page says "CORN by Mandy", and the stack of paper is gone.
Episode XCII - S5-E1
Deliberate mistake: Jack stops his motorbike beside a stream, and Jack's weapon strap crosses over his left shoulder when he walks over to have a drink. The problem is when we see Jack's reflection in the water, his image is not reversed; note the strap is still crossing over Jack's left shoulder. (00:07:20)