When a person makes a mistake and it's stupid. A STUpid mistake applies to people with the nameStu or Stuart.
Stu: "I tried to cut a guy off in traffic, but he wouldn't let me, so I started tailgating him, but then he slammed on his breaks, ran up to my car and scared the sh*t out of me."
When you do something or you don't want them but I don't want you so your the best mistake. I didn't want you before but now I do and it also mean you did something you regret and now you don't regret it anymore