Ross Geller - Friends
This one could go either way. Since Ross and Rachel were "on a break," Ross took that as a break-up and slept with someone else that night. Did he think he was cheating on her? Probably not. Was it okay? Absolutely not. As Rachel said, Ross may have thought their relationship was dead, but he had a great time at the wake.
Kit Walker - American Horror Story
When Kit's wife was abducted by aliens, he was charged for her murder and put in an insane asylum. While there, he fell for a fellow patient named Grace but realized at the end of the season that his wife was still alive.
Karen Walker - Will and Grace
After her husband dies Karen enters a new relationship. But it turns out that he had been faking his death the entire time.
Laurel Lance - Arrow
Oliver was cheating on Laurel, so this one feels like it shouldn't count. But, the two never officially broke up as Laurel thought he was dead, and she ended up starting a relationship with his best friend, Tommy.
Finn Collins - The 100
Was it really an accident? We're not so sure. After being sent to the ground, Finn falls for Clarke and the two start a romance. It's later revealed that Finn has a girlfriend in space, but he was under the impression that he was never going to see her again. However, he didn't hold out for too long.