There's been bangers but the top so far is a peer in fact who didn't get his way about upcoming changes such as a change of branding, and nuked his account (imagine what this does to interfaces messages when x person did this), suddenly withdrew without handing off any access to cloudflare (among other things) leaving the tech group scrambling and almost resulting in the other main tech quitting, left several groups of users and volunteers hanging and related messes. This was a large project, a community hosting communities so while there are related stories that one's probably the big one. It's also the most important, in terms of things getting too dependent on one individual who isn't the straight up owner (this is a shared group with an organization) and runaway burnout as he was vastly overstretched anyway.
Somehow the place not only runs but this isn't the first time something of the sort has happened, and it's still getting on fine.
I can't say I have the experience directly as the owner dealing with the subordinate, at least anything major enough to note (I have come in, cleaned up house and pissed some people off that way, up to the former staff becoming vagrants and spamming/trying to sneak back in - think I made the right call there?). I rarely hold that title anyway though.