Why "less then" leaves linguists less than impressed

What kind of error is it?

grammatical error

Why do people make this mistake?

People often confuse "then" and "than" because they sound similar and have similar spelling, but they serve different grammatical functions. "Then" is an adverb often related to time, while "than" is a conjunction used for comparisons.

What is correct?

"Less than" is correct because "than" is the conjunction used to make comparisons.