3. Hailey Upton and Adam Ruzek
This one is just strange to think about. Upton and Adam Ruzek (Patrick John Flueger) are so closely tied to other romantic partners on Chicago PD that when they decided to go on a few dates in season 6. They kept their romance under wraps, but the whole felt very contrived, like when Joey and Rachel dated on Friends.
Related: Chicago PD still has to work on Upton's arc
Eventually, Upton and Ruzek broke things off, and got back with their respective "endgame" partners, leaving this little detour a bizarre footnote in the romantic history of the IU.