Based on your comments, it seems likely that your keyboard navigation extension is to blame. When you make changes to your Favorite Tags, the list is persisted to the database by calling
.text() on the
#interestingTags page element and sending that string back to the server. If the extension is inserting content into this element, such as the index number of the tag links (noting the values and sequence in your screenshot; the tag links there should be somewhere between the 550-650th links on the page), this information will get inadvertently saved as well.
Since the list isn't refreshed from the server when you add tags originally, you don't notice the problem then - it's only on subsequent loads when you request the messed up tag list from the server that you see something's wrong.
It's very possible that this only happens in certain circumstances, depending on the behaviour of the extension. If you have an installation link, I'll check it out in more detail, but based on what you were seeing this seems like the most likely explanation.