Amateur Mistake
(Noun) An error resulting from a profound lack of judgement - usually on an amateur scale.
Fred: Yeah, I got ridiculously drunk and got with my ex-girlfriend last night.
Johnathon: You didn't! That's an amateur mistake!
Alex: Oh no - I left my graphical calculator in radians for all those questions!
Christopher: Oh dear - amateur mistake!
Johnathon: You didn't! That's an amateur mistake!
Alex: Oh no - I left my graphical calculator in radians for all those questions!
Christopher: Oh dear - amateur mistake!