Last night, before heading for bed, I noticed that there was an update available for OS X on the iMac, and also for iTunes. Despite the late hour I decided to do the update anyway. OS X updated pretty smoothly (albeit with some rather unhelpful progress bars that appear to give estimated times that have no relation to reality), as did iTunes.
I was then told that there was an update for my iPod as well (all part of this new thing where Apple have invented Spotify, obviously). I let that start doing its thing and that's where things got odd. First it started the update and the iPod appeared to insist that it wasn't plugged into the iMac, even though it was. Then I gave it a second go (after unplugging it and plugging it in again) and it all seemed to go through just fine, only...
...while the iPod was pretty sure it was now running iOS 8.4, iTunes on the iMac had other ideas:
The following morning iTunes kept insisting that it needed to do the update so, even though the iPod was obviously up to date, I let it do it anyway. After it'd gone through the update process again it still insisted that the iPod was running 8.3 rather than 8.4. Until, that is, I unplgged it and plugged it back in again.
So now it all seems fine. I just had to do a variation on "have you tried turning it off and on again?"
Once again I find myself running into things on a Mac that are very common elsewhere, on other operating systems, and which Mac owners would often have you believe weren't an issue.