1.First off, you can see the George Foreman grill that Michael burns his foot on — because he likes to wake up to the smell of bacon — in the episode "Dinner Party."
2.You know the teapot Jim gives Pam for Christmas? She uses it later in the Finer Things Club.
3.The lawyer Michael hires in Season 2 is the same lawyer Michael mentions when he and Dwight throw a watermelon off the roof and it hits a car in Season 3.
4.You could definitely already know this one, but it's amazing. Michael's girlfriend Carol — who broke up with him because he photoshopped himself into pictures of her family — has a tiny cameo in Threat Level Midnight. She's the Realtor of the real-life house Michael Scarn lives in.
5.Dwight and Kevin absolutely love fireworks in Season 2, and they're outraged by a potential lack of firecrackers in Season 6.
6.The graphic design guy Pam talks to at the job fair in Season 4 is the office IT person later in the show.
9.Another item at the garage sale is the stuck-to-the-wall painting from when Jim bought Pam their house. They finally got it off the wall.
10.In Season 3, Michael tells Jan about the kind of love he wants, and it includes "ketchup fights." He puts a ketchup fight in his movie Threat Level Midnight.
11.This one's meant to be obvious. Elizabeth, the dancer at Bob Vance's bachelor party and the recipient of the check made out to "Science," shows up in the finale at Dwight's bachelor party.
12.Also not suuuuuuuper subtle, but Angela's babies-as-adults poster that Oscar hates — and complains about to Toby — was, in fact, a Christmas gift from Toby.
13.When Jan hands Michael their relationship-disclosing agreement for the company, he says he's going to frame it. He does.
14.Remember that time Michael, Dwight, and Jim drove to the Utica branch and Michael and Dwight tried to steal the copier? Well, the Utica copier is padlocked when we see it in Season 5.
15.Dwight tells us a very ominous Schrute wedding tradition, and he and Angela do it when they get married seasons later.
16.The same person teaches a diversity seminar in Scranton and in Stamford. It's also the exact same seminar.