Incorrect author name being assigned to posts

The strangest thing has been happening. I am logged into WP (as admin), and when I save posts and pages, my nickname is now being applied to the account – the name of one of the other users is instead. This has been happening for the past 24 hours. I believe I updated the blog to version 2.6.3 just before this started to happen, using the auto-update plugin. The update went off without a hitch.

Does anyone know why this is happening, and how I can fix it?