A private update checker I’d built took a WordPress site down the moment a plugin was activated. The culprit was load order. Active plugins register their update slug before init, then my must-use plugin tried to register the same slug again on init, and the update library reports a duplicate with E_USER_ERROR, which is fatal with debugging on. The fix was one line: ask the library’s slug-in-use filter first and skip anything already claimed. Two bits of code that each “just register the slug” are fine, right up until they both do it in the same request.