1. Jim and Pam's clown poster in their home.

All spoilers aside (it's been out for ages), when Jim and Pam move in together, they notice a picture of a creepy clown that is "apparently integral" to the design of the house. With the confirmation that it cannot be moved from its place on the wall (and a short clip of Jim trying to death it unsuccessfully), there's no way that it can appear elsewhere. But it does.

During the office garage sale in season 7, the clown poster appears in hindsight of a scene. When characters Andy, Darrell, and Kevin are playing a board game together, the poster can be seen in front of the Halperts' table, ready to be sold.

2. Pam's mom, Helene.

In season 2, Pam's mom visits the office to see her and spend some time with her. She doesn't have a cell phone or an email, so it makes it more mysterious of when she may appear (but honestly: no email or cell phone, how does she keep in contact with her daughter?). When she finally makes an appearance, the audience of the show can observe what she looks like.

Later on, Pam's mother makes a reappearance in the show when she shows up at Pam and Jim's wedding. It is very clearly a different actress. So much so that Michael Scott, branch manager, starts to see her and date her. NBC thinks just because they're 2-3 seasons apart, their audience won't pick up on that. Nice try.

3. Jim and Pam's wedding dinner.

Very quickly flashed across the screen, when Pam and Jim host their dinner party the night before the wedding, the sign with their last names combined is spelled incorrectly. It reads "Halpret" instead of "Halpert".

4. The IT guy, Nick.

via GIPHY

In an earlier season, while Jim and Pam are still just dating, Pam, Darrell, Oscar and Michael visit Pam's alma mater high school for a job fair. When Pam decides to go and roam a bit, she finds a booth for graphic design. As she sits down to start filling out an application, we notice and take in the features of the person sitting there introducing the position to her. This same actor appears later in the show when Sabre buys out Dunder Mifflin. His name is Nick, and he is the IT guy for the office.

5. Jim's signature on Meredith's cast.

When Michael runs Meredith over with his car, she recovers in the hospital and returns to the office in a later episode with a cast around her waist for her hip. She singled Jim out to sign her cast and insisted on it. Noticeably, when Jim signs his name, it seems as if he writes "John Krasinski", the actor's real name, instead of "Jim Halpert".