Lately two are getting on my nerves:
In games - Amnesia: I get it, you need to somehow make the gamer experience the story from a humble beginning, and then develop as the character develops. But there are other ways, take the Princess Bride route where the main character was a simple farm boy at the beginning. Amnesia is a simplistic excuse, a lazy resource. Just stop using it!!!!
In movies - Femme Fatale: every movie apparently has a woman who fights alongside his male counter part...even those supposedly based based in historic events, in historic periods where women were never in the front line (in most cultures). They have even go as far as taken female characters that are supposed to be guides, or mentors. I am not against the idea, but sometimes is forced (King Arthur comes to mind here) and adds nothing to the plot.
oh one more, though not exactly a plot device:
cultural historical inaccuracies. If you are making a movie about a fictional event, but located in a specific time frame, do your research. I do not need to see a viking using a conquistador's armor, or a roman gladiatorial helmet for that matter (13th Warrior).