I work in the cluster of office buildings between Uchisaiwaicho (Toei) and Kasumigaseki (Metro). I know a number of people who tell their company they are buying a Toei commuter pass, which is more expensive, and then buy a less expensive Metro commuter pass with the money.

This sort of thing is also really common in northern Kansai, when you generally have a choice of three commuter lines (JR, Hankyu, and either Hanshin or Keihan) to get to Osaka. Hankyu is significantly cheaper, so many people will claim to ride JR, commute on Hankyu and pocket the difference. If you're REALLY lucky, you can add in a fake monorail segment somewhere.