A Rookie Mistake
Meaning: An error or blunder typically made by someone who is inexperienced or new to a particular activity or job. The phrase implies that the mistake is due to lack of experience or knowledge, rather than a lack of ability.
Examples:
- Forgetting to save the document before closing it was definitely a rookie mistake—now I have to start over.
- He made a rookie mistake by not checking the weather before planning the outdoor event, and it rained the whole day.