"Perception is reality" I suppose. Generally speaking I think of a mistake as being an unintentional act. And yet I guess it's possible to commit an (intentional act) that leads to "unintentional consequences"!
In my opinion, an intentional action may be deemed a mistake if it occurs as an error in judgment. Sometimes circumstances present facts upon which someone may make a judgement call followed by a series of actions, only to realize at a later time that all such actions were misguided. A typical example is when a doctor misdiagnoses a sickness because of unreliable diagnostic modalities. This can be a mistake. It can also happen in relationships. Some times things happen under the influence of inadequate or inaccurate information.
However, for something to be excused as a mistake, the perpetrator must demonstrate remorse and explain how they were mislead into making the "mistake"
"an error in judgment" is something to consider. However since this question is posed under relationship problems it most likely would not apply. Most people know what their mate expects of them or have taken vows. Impulsive decisions = selfish