When Michael is talking with Carole Stills, she says that Michael proposed to her on their 4th date (Diwali (2006)). However, right after the proposal, Carol states that it was their 9th date, not 4th.
Michael is reunited with Jan and her daughter, Astrid. In Baby Shower (2008), Astrid was a mere month or 2 old. However, in what is only a 2 year passage of time, Molly Burk (as Astrid) is obviously much closer to age 5 than the 2 years and few months that Jan's daughter should currently be.
While discussing the risks of STDs in the conference room, Erin and Gabe are seated against the back wall of the conference room, but there is a window behind them. That wall in previous seasons has no window.
When Andy is writing Pros and Cons on the whiteboard, the written appearance of everything changes a few times - most likely due to multiple takes.