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Fixed expressions

By mistake or of mistake?

The standard English expression is by mistake. Of mistake is not idiomatic in modern English.

Quick answer

Correct expression

Use by mistake when something happens accidentally. Do not use of mistake as a fixed phrase.

I took your bag by mistake.
She sent the email by mistake.

Choose the pattern by meaning

These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.

1
by mistake

Describing an accidental action

Use by mistake to mean accidentally or unintentionally.

He deleted the file by mistake.
2
mistake in + activity

Showing where the error happened

Use mistake in to refer to an error within something.

There was a mistake in the report.
3
mistake about + subject

Showing misunderstanding

Use mistake about when someone is wrong about a topic.

You are making a mistake about the schedule.

See the difference in context

The same subject can take different constructions because each sentence answers a different question.

Construction
Example
Meaning
by mistake
I called him by mistake.
accidentally
of mistake
The phrase of mistake is not standard English.
incorrect fixed expression
mistake in
There is a mistake in the calculation.
an error within something
mistake about
She was mistaken about the time.
wrong understanding of something

Common mistakes

Some combinations appear in real text but do not mean what learners often assume.

Using of mistake

English uses by mistake for accidental actions.

✕ I took it of mistake ✓ I took it by mistake

Using mistake as an adjective

Use mistaken for people who are wrong.

✕ I am mistake about that ✓ I am mistaken about that

Missing by in the fixed phrase

The preposition is required.

✕ He pressed send mistake ✓ He pressed send by mistake

Confusing by mistake and on purpose

These expressions have opposite meanings.

✕ She broke it on purpose when you mean accidental ✓ She broke it by mistake

Quick practice

Choose the expression that matches the intended meaning.

1. I deleted the message ___ mistake.
2. There is a mistake ___ the document.
3. The phrase ___ is not standard English.

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Frequently asked questions

Is by mistake formal or informal?

It works in both formal and informal English.

Can I say of mistake?

No. Modern English uses by mistake instead.

What is another way to say by mistake?

You can say accidentally or unintentionally.

of, about, against, to or for racism? of, about, for, in or on friendship? of, about, for, on or to thing? of, about, for, to or with sex? of, about, on, to or for moon? "of access" or "with access"? "of accommodation" or "for accommodation"? "of accuracy" or "for accuracy"? "of action" or "in action"? "of actor", "with actor" or "by actor"? "of address" or "to address"? "of administration" or "in administration"?

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