Do you use the Store and iDevice syncing parts of it at all? The Store always has me running around in circles unless I know exactly what I'm after and can search for it directly. The back button and breadcrumbs often behave contrary to expectation, and for some reason there only is a global search. The store also seems excruciatingly slow; I assume the servers are all located in North America and the content isn't optimised for transcontinental latencies.
iDevice syncing is full of unhandled corner cases which lead to crashes, inconsistent state, nonsense error messages, or lost data. It's also really easy to accidentally delete stuff off your device if you don't have the same data locally. I probably get the short end of the stick for being a developer: iTunes and XCode have a kind of love-hate relationship. That's far from the only source of problems though, I've seen all kinds of weirdness outside the context of iOS development.
Even outside of this, there are some embarrassing hiccups, e.g.:
- The UI becomes extremely unresponsive while importing media - it seems the import runs in the UI thread, or otherwise causes the UI thread to block.
- If you're editing metadata for a file while it's playing, there's a brief pause while saving.
There's no excuse for either of those, and seem the sort of thing a decent programmer with knowledge of the codebase could fix in a day or so. (I'm aware that ID3v2 is typically stored at the start of the file and requires the whole file to be rewritten; notifying the streaming playback thread of the byte offset shift and new file descriptor is however not rocket science)