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Source: https://writers.stackexchange.com/a/12529
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Off the top of my head, solmnambulation and memory-impairing drugs are probably the easiest and most probable. Human memory is somewhat frail. In real life this generally impacts the accuracy and availability of memories, but yes, under the right circumstances people can fail to remember having done something. Many things affect memory, and with a little research I'm certain you could find at least ten scenarios that I'm not even thinking of right now.

I am working off of the assumption that you're not interested in trauma-related memory suppression.

1) **Sleep-walking** is not uncommon, and people who sleep-walk are able to perform all sorts of complex actions while immersed in slow-wave sleep. Look up homicidal sleepwalking on Wikipedia, it's a hoot. As a bonus **some medications can cause sleep-walking** as a side-effect, including a certain popular insomnia aid.

2) Certain drugs can cause memory loss or poor memory formation. One such is **GHB** which has gained some notoriety for its use in date rape. It's also occassionally found to have been used in cases of mugging and the like, and it can be difficult to detect. There are certainly other drugs that affect memory, too.

3) Just in case, there is always the possibility of **post-traumatic memory loss**. In fiction (especially movies) one commonly finds this trope in conjunction with hypnosis. Memory repression hearkens back to Freudian psychoanalysis. This choice has a strong whiff of pop-psychology, and hypnosis therapy in particular is _highly_ controversial.

Hopefully these options can give you something to chew on.

Original score: 6