Formal:
* Unwitting blunder: This emphasizes the lack of intention and the negative result.
* Inadvertent mistake: This implies a lack of awareness or carelessness.
* Innocent misjudgment: This suggests a good intention but a poor understanding of the situation.
Informal:
* Dumb luck: This is sarcastic and humorous, implying that the stupidity led to a fortunate outcome.
* Brain fart: This is a casual and humorous expression for a momentary lapse in judgment.
* Oops, my bad: This is a self-deprecating way of acknowledging a mistake.
* Out of one's depth: This suggests a lack of knowledge or understanding.
* Missing a beat: This implies a failure to grasp the situation.
Other options:
* Unintentional gaffe: A social blunder.
* Careless mistake: A mistake made due to a lack of attention.
* Foolish error: A mistake made due to a lack of judgment.
* Clumsy mistake: A mistake made due to lack of coordination.
The best phrase to use will depend on the specific situation and the tone you wish to convey.