It just happened to me that unwatching a page failed, because the token changed for some reason. I just got an error message of the kind "something went wrong" which doesn't help normal users (I could look in the JS console to find out what went wrong). Even if this shouldn't happen too often, MW should be more helpful. Possible enhancements:

• Add some hint how to fix it to the error message: "Reload the page and try again."
• Unbind the AJAX watch things when an error occurs (the non-AJAX token will probably be invalid, too, so the user will need to confirm the action a second time).
• Use action=tokens to get a new token and re-try automatically.

Version: 1.22.0
Severity: enhancement