There's nothing that YOU did wrong... it's just genetics - unfortunately, you're going to get a lot of comments here that say autoflowers are terrible, don't grow them, yada yada... but even photoperiods will once in a while have one seed that just goofs up... pretty normal, pretty irritating, pretty puzzling... just treat each plant as an individual and appreciate what she gives you... that particular plant might make some very nice edibles for you!
Good luck!