I still use iTunes sync my iPhone periodically to my computer (mainly so I can retrieve photos off the phone). Every time I do, I get to watch this:
It copies several hundred unchanged songs back onto my phone. This is apparently a bug. A known bug, known for years, that Apple just won’t fix because it’s Apple being Apple. Screw them.
(This bug is perhaps unrelated to a separate problem — honestly, a bug — wherein someone decided it would be a good idea to change the file’s “modification” time every time a song was played. This is an idea so stupid I simply cannot imagine how anyone thought it was clever.)