The word "bjørnetjeneste" has two meanings. The original meaning: To do someone a favor that actually does the opposite of helping them. Example: If I make my little brother's homework every day I do him a favor, but he's not going to pass his exams because of it.
The newer meaning: A huge favor.
Danes will most likely know both meanings, but adults will probably only know the first one, since it's the original and intended meaning of the word.