1. Everyone from both groups of friends, except for Barney, lives in the main apartment at some point.
2. Ross and Chandler meet each other in college and become best friends, and Ted and Marshall do the same thing.
3. Mike breaks up with the girl Ted dumped on her birthday when he started dating Phoebe again.
4. No one in either group knows what Chandler or Barney do for a living.
5. In a flashback, Joey tries to get Monica to like him by stripping off. On HIMYM, Barney and Ted use “The Naked Man” to get a girl’s attention.
6. The same person plays Ross’s mother, Monica’s mother, and Lily’s grandmother.
7. Joey practices his lines for a new role with a pineapple, and “The Pineapple Incident” has been a running joke on How I Met Your Mother for a long time.
8. Ted and Ross both feel like they have to correct their friends’ grammar all the time.
9. Both Phoebe and Barney use fake names a lot.
10. One of Chandler and Monica’s best friends also married Marshall and Lily.