Denon DN-HP1000 Swivel
My beloved headphones first developed a crack in their left swivel about a year ago. The crazy glue held up for some time, but ultimately they blew apart when I was out for a run (yes, I ran with these giant headphones on). As I looked frantically for shards of headphone in the bushes, i thought "Wait a minute, I can just print a new one!"
This particular part ended up being much more complicated than I'd anticipated. It holds everything together on the left side and the build logic that Denon designed is impressive (and frustrating). It also includes features that limit rotation in two axes. I recommend partially disassembling the other side for reference.