The problem seems to have resolved itself last night.
I'm running a chat room, and a couple days ago I started seeing my own username appearing to pop in and out of the chatroom. The chatroom database has a table listing users currently online. I can TRUNCATE the table, deleting all records, and the table may briefly show zero records, but then immediately the username pops in again. I have deleted that user but somehow it is still showing up. I created a new username for myself and now the old one is just sitting there in the chatroom, no longer popping in and out because it's not clashing with my actual logging in under the same name.
I'm guessing (from what little I know about the webhosting side of things) that the session with this username somehow got stuck and is still open. Is there any way to do a reboot of sorts which will close everything that's open so I can start fresh?