Why "contains of" is an linguistic slip: the art of precise language use

What kind of error is it?

It's a grammatical error related to the incorrect use of phrase construction in English.

Why do people make this mistake?

People often make this error because they try to construct a phrase that combines elements typically not used together in English. The phrase "contains of" mistakenly attempts to pair "contains," a transitive verb that directly takes an object, with "of," a preposition, which is unnecessary and incorrect in this context.

What is correct?

The correct alternative is "consists of" when you want to talk about the components or elements making up something. "Contains" is used without "of" to indicate inclusion or holding within.