Continually forgetting to do something is another sign of passive aggression to watch out for. Some people are generally forgetful, disorganized, or easily distracted, but there are limits to how much forgetting you should put up with if people are otherwise mentally healthy.
The reason people deliberately forget, or deliberately do something that they know will make them forget, is that they really don’t want to do what they have promised you or what is expected of them. Yet they also don’t want to tell you that they don’t want to do what they promised or what’s expected. For example, if you have a standing agreement with your live-in partner that you put dirty dishes in the dishwasher immediately after using them instead of letting them pile up on the counter, but your partner frequently leaves dirty dishes behind, then it’s probably a sign of deliberate forgetting—or at least that’s a reasonable conclusion if you have confronted them, and it keeps happening.