You were a mistake
1. A wicked insult to scar or otherwise mentally harm your brother,enemy,troll,fag,loser,low-life or whoever it is you are arguing/disagreeing with.

2.Something you say to your kids to make them feel 'special'.Popular with fathers,mostly.Similar to 'you were an accident'.
1. (Random Youtuber watching one of Gligar13Vids reviews,posting a comment) : Damn,dude.Either you was dropped as a baby or you were a mistake!

Gligar13Vids : D: ....

2. Daddy: Why the fuck did you knock over that vase?!Why did my bitch even have you?!You were a mistake!!

Kid: (sarcastically)Gee,thanks,Dad!