Please share Denny
Years ago, on rec.crafts.brewing, this very topic came up. I started just rinsing out kegs (if even that) before refilling them. My theory is that the low pH of the beer, along with the alcohol content, made the beer resistant to infection. The flaw was "resistant" didn't mean "impossible"! It worked fine for a few months, but then I started noticing off flavors in beers where the kegs had been reused several times without cleaning. It finally went away when I tore all of those kegs down and gave them all a thorough cleaning and sanitizing. Of course, it's possible that I had a run of bad beers caused by something else, but circumstantial evidence pointed to the lack of keg cleaning. Since then, I always clean and sanitize kegs and the problem hasn't reappeared.