I've found a reproducible issue while editing my "Favorite Tags".
- While logged in, open two browsers windows and visit a StackExchange site.
- In the first window, click the "edit" link in the Favorite Tags section.
- Type in a new favorite tag, and click "Add".
- Switch to the other window, and add another new tag.
- Refresh each page.
After refreshing either page, both new tags will be displayed in the order they were added.
Only the second tag is saved, and the first one is overwritten. It appears whatever was visible at the time of adding the newest tag will be persisted.