Wanting to delete a tag in its entirety (not just the references) and lacking a tag management page, I proceeded to filter by tag on status:any issues.

Having deleted the tag in the first matched issue, a refresh on my original view_all_bug_page.php resulted in an error:

Application Error 0002200 - Cannot find tag by that name

This looked to be caused by the existing filter which was set to filter by tag 'foo', which no longer existed, ultimately confirming the error.

This has been somewhat corroborated by nuclear_eclipse, after reporting and discussing it on IRC.

A logout -> empty cache -> restart browser sequence is sufficient to alleviate the symptoms.

As far as resolution is concerned and with my limited knowledge of the codebase, there may be 2 or more possible solutions:

1 - Reset entire current filter in its entirety after tag deletion logic
2 - Reset tag list in filter after tag deletion logic
3 - Gracefully display the missing tag in the filter area but show no matches

I would suggest (3) given the user knows they just deleted the tag, so 'no matches' is also an expected output, confirming the operation.