So today iTunes asked me if I wanted to update my iPhone's operating system to iOS 10.2.1. I said yes, but iTunes replied with an unknown error '-50', twice (because the dialog showed up twice for some reason). After these errors iTunes shows an error saying it can't find my iPhone, even though it's firmly connected to the computer. Also, my iPhone is listed in iTunes' devices, so either the device list or the error dialog is lying to me. When trying to sync from the device view I get the same error. I unplug and replug the cable and now I'm able to sync again, so apparently iTunes just can't keep track of connected devices.
So I updated. And guess what? Shortly after, with my iPhone unplugged from the computer, iTunes asks me if I want to update it yet again.
I wish someone at Apple would take note of how painful using iTunes is getting year by year. More issues keep arising among new features than what the developers are able to fix. It's been the trend for past several years, and seems to continue that way.