TYPO3 Core

Editors rename pages and slugs back and forward.
For example: "/slug-actual" to "/new-slug" and later on back to "/slug-actual" because someone told them, whatever. Its kind of usual in editors life :-)

If slug updates are not reverted in such cases (and this is kind of tricky, as you maybe will asume, a redirect will make sense probably, and you not remember your renaming back and forward), this creates endless loops and editors don't get a information from the system, that they have created a endless loop. But this would be awesome.