Steps to reproduce:
Go to the excerpt history of a tag.
Click on edit.
Change the excerpt of the tag.
Save your edit and you'll see that "page not found" page.
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While I can reproduce this issue, we aren't going to fix it right now, due to time constraints.
The "edit tag wiki" code path is entirely separate from all other post types' edit path - so those edit links on a revisions page do not have all the logic needed to validate a tag wiki edit.
A refactor of our edit code is something that's near and dear to my heart and we will address this shortcoming then, but for now, I'm going to remove the edit and rollback links from tag wiki revisions pages for non-moderators.
The workaround is to use the "Edit Tag Wiki" button at the bottom of any tag wikis to improve either the excerpt or body.