We had a married couple working at the company, both engineers and neither reporting to the other. The husband was promoted to a managerial position where his wife would now become a report, not direct, but still in his chain-of-command. That is against company policy so they were given some options to resolve this. The wife ended up with a lateral transfer into another division.
Awhile back one of my co-workers got involved with a contractor. Apparently the relationship went "bad" at some point and one of the parties decided to get HR involved. Both were fired immediately.