One particular writing technique that I absolutely despise, is the convenient amnesia a character must suffer in order for the plot to be resolved. Or "must" suffer, since I tend to see it as little more than laziness on the part of the author, instead of a legitimate narrative device as part of the denouement.
Convenient amnesia takes the form of, for example, the hero having his memories erased (by injury or by magic) in order that some secret be maintained, or disaster averted. I think this is always a poor choice for the author to take, because in my mind there is never a lack of an alternative that doesn't involve abusing the protagonist like that. Discovering what that alternative is is just part of the writing process.
The first time I recall encountering this device was in Raymond Feist's novel Faerie Tale, an otherwise enjoyable book that was ruined for me at the end when the protagonists are made to forget their adventures in the land of the faeries. More recently, in the Series 4 Doctor Who climax, companion Donna Noble is made to forget everything that happened while she was traveling with the Doctor, so that her memories wouldn't kill her. Now theoretically, I suppose, ridding someone of lethal memories is a good way to save their life, but only if you've already written yourself into that particular corner.
Not only is it a lazy device, but I even think it's disrespectful of your characters. No, they're not real; their memories aren't real; blah blah. But the thing is, they're sort of real. They're as real as it's possible for you to make them, so don't take away the only thing that made them exist in the first place -- the things than happened to them while they existed.