EDIT: Looks like the miraculous return of this bug, which was apparently fixed over a year ago.
I recently changed my username to Arbiter on an SE sites and noticed it had changed from Purmou to Arbiter on StackExchange.com.
I then clicked on my username to find that my profile was still displaying the old one. (This is far too serious for freehand circles, so I went with freehand emphasis bubbles and blocky question marks!)
It turns out that the header is only different when you also change the email on that account. I can't actually test this since I can't reset my username for another 5 days.
Which leads into the other problem (and suggests that it might stem from the same root issue): after changing my email on the same site, I now receive tag subscription emails in the inbox of that email, and not the one I have set on the site where the subscribed tag lives.
In other words, Stack Overflow tag subscription emails are being sent to the email address on my newly changed account. That is the only account I use that email address on.
I have already tried syncing with my oldest account to no avail.
So, what could possibly be causing these inconsistencies?