This is an old story of mine but many years ago, before internet forums and such I made a batch of malt vinegar INSIDE a keg of crappy pale ale. It sat in the keg, in my garage, for a year before i bottled the vinegar. After I racked the vinegar to my bottling bucket, I cleaned the keg with PBW as usual, sanitized as usual and at some point later filled the keg with beer. Over the 15 years since I made that vinegar in the keg I've probably served 50 batches of beer in it without ever even replacing the o-rings. I have no idea which keg it was but in 20 years of brewing I've never had an infected batch of beer, even in that keg.