Not a seasonal anime but...
I rewatched K-On over the past couple days; and in the first episode, during Yui's run to school (in the first two minutes), there are cameos of Mugi, Ritsu, and Mio in the background.
As for the rest of the series it was worse than I remember, though it's still one of my favorites, as well as my first. When I last watched it, the episodes about entrance exams, university admissions, etc. worried me because I was near the end of my junior year of high school and had no plans at all for the future. Now that I've been a NEET for five years those episodes were much more worrisome...