rookie mistake
The phrase "rookie mistake" typically refers to an error or blunder made by someone who is new or inexperienced in a particular field or activity. It implies that the mistake could have been avoided if the person had more knowledge, skill, or understanding gained through experience.
An idiom is a phrase or expression that usually presents a figurative, non-literal meaning attached to the phrase. Does that sound fun? It can be!
Play Just Saying - the word game that makes learning idioms a good time on your mobile phone or tablet.