Definition of Schoolboy error...

"getting the basics wrong" ... especially when you should know the answer

The best answer to literally anything

Schoolboy Error: (n.), (der. old English), A mistake, often trivial, clumsy, dimwitted or easily avoided. Generally shameful in hindsight, but without cause for embarassment, owing more to circumstances or instantaneous bad luck, rather than serious incompetence of behalf of the maker. Derived from the "trial and error" psychology present in Victorian-era English public schools.

For a person of experience to make an error in a situation where she should know better.

Making the most basic of simple mistakes, worthy of ridicule, particularly when you should know better

school boy error is when you make a simple yet stupid mistake. you feel stupid after you've done it.

The most basic of errors. An error so simple, and avoidable, that anyone with an ounce of credibility or experience in that field should not be commiting it. A mistake that should only be made by school boys still in training for that particular task. Especially used when referring to playing games of pool in pubs, or occasionally during snooker. Schoolboy error can be heard ringing out by jeering onlookers if for instance a participant missed a sitter = he should have potted it as it as so simple, that thereforehe is reduced to the level of a lowly school boy playing his dad for the first time.