I certainly don't consider myself an expert on homemade vinegar, but I did once make a wonderful batch of it unintentionally.
As some of you may know, I'm a home winemaker. At any given time, I have 100-150 gallons on the go in my cellar. The vast majority of it turns out great, but there are nevertheless the occasional.... well... mishaps.
Back in 2006 I made a batch of Zinfandel that, for whatever reason, I wasn't totally thrilled with. It was drinkable, but that's about the best that could be said for it. It was thin and didn't have a lot of body and color. So I set the 3-gallon carboy under my workbench with the intention of dealing with it later (the winemaker's equivalent of sweeping things under the rug).
The container had a water-filled airlock on the top. At some point, I'm assuming months later, the water evaporated from the airlock, leaving the wine inside exposed to air, although the airlock cap continued to keep out insects and other nasties.
It must have been several months later that, while cleaning up and reorganizing, I discovered the neglected carboy with the empty airlock. It had a waxy layer of mycoderma (vinegar) bacteria growing on the surface of the wine, which, in retrospect, probably helped protect it from total oxidation. My first thought was that maybe this was still somehow salvageable. I instinctively took the top off and had a whiff. Vinegar. Argh.
I almost dumped it out, but after making a couple of calls and being reassured by a professional winemaker acquaintance that it was indeed safe, I tasted a sample. It was fantastic. The best red wine vinegar I've ever tasted - much more concentrated, complex, and smoother than most commercial products. So I strained it and cold stabilized in the garage over the winter to let any excess tartaric acid settle out. The next spring I ran it through my filtering setup and packaged it up in some nice dessert-style wine bottles. I ended up with 30 375ml bottles.
In addition to the dozen or so bottles we went through ourselves, I also gave quite a bit of it away to friends and family. Everyone really enjoyed it and asked when I would be making it again. Up to this point, I had only told a few people that it was an accident.
Now that it's gone, I wish that I had saved some of that vinegar to make more. The stuff I buy at the store now just doesn't hold a candle to it.