I've found a reproducible issue while editing my "Favorite Tags".
To reproduce:
- 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.
Expected result:
After refreshing either page, both new tags will be displayed in the order they were added.
Actual Result:
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.