I saw this on a blog yesterday.

Problem:
A contraction appears where a pronoun is required.

Explanation:
The full sentence was something like, “I work with a guy who’s service goes down every day for an hour and a half.”

The writer used W-H-O-apostrophe-S, which is a contraction of the word “who” plus the word “is”.

What he should have put was the pronoun “whose” — spelled W-H-O-S-E — which is the possessive case of the pronoun “who” — just as “his” is the possessive case of the pronoun “he”.

Solution:
“… a guy whose service goes down every day …”