When someone cheated and had sex with other people beside their SO, you will often hear it was a mistake. Is there just thing as a mistake? Why?

My sister was married for only 11 months and then got divorced because she ended up sleeping with one of her male friends while still married. She said she was drunk and it was a mistake.

When someone cheated and had sex with other people beside their SO, you will often hear it was a mistake. Is there just thing as a mistake? Why?
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