Today I accidentally created a self referencing synonym ended up crashing that tag's page until I found a way to remove it. My procedure was:
Propose that Y map to existing tag X using the standard synonym suggestion interface.
Merge X into Y (including checking that box at the bottom) of the moderator tag merging system.
Go to the full synonym list and approve the suggested synonym from before, only now it was a self referencing synonym.
I and other users then couldn't get to the page /questions/tagged/Y (a redirect loop error).
The issue was resolved when I removed the self referencing tag from the main synonym page.
I know that this isn't likely to happen often, but perhaps something should be in place to prevent it just in case.