Why "old fashion way" isn't quite right

What kind of error is it?

It is a phrase error due to incorrect usage or outdated colloquial expression. The typical phrase intended is "old-fashioned way", which refers to doing something in a traditional or time-tested manner.

Why do people make this mistake?

People might make this error due to a misunderstanding or mishearing of the phrase over time. Sometimes casual speech or regional dialects contribute to the adoption of non-standard phrases.

What is correct?

The correct phrase is "old-fashioned way". It properly uses the adjective "old-fashioned" to describe the manner in which something is done, emphasizing tradition or antiquity.