1. Sylvie Brett and Joe Cruz
This one is so strange. Sylvie Brett (Kara Killmer) was new to the 51, and she succumbed to the charm of one Joe Cruz. They dated for a brief time, but Brett ultimately called it quits in season 3 when she realized that they were not on the same page.
Things have obviously remained fine between Brett and Cruz, and they are good friends, but the notion of Brett, who is engaged to Casey, and Cruz, who is married and has two children, being a couple for any period of time is such an oddity that it may catch some viewers off-guard during a rewatch.
Ultimately, the Chicago Fire writers made the right call.