Why "some one" is written as one: breaking down a common mistake

What kind of error is it?

Spelling and spacing mistake

Why do people make this mistake?

People often mistakenly think that "someone" needs to be separated into two words, similar to phrases like "some time" and "some day", which can be correct in different contexts.

What is correct?

The correct form is "someone", used as a single word when referring to an unspecified person.