Last night I bottled four quarts of my crabby cider (apple cider with the addition of some crab apples). It had reached 1.004 at 17.5°C in my sample tube, which should give an ABV of about 4.5%. However, when I tasted a sample from what was left (not enough to fill another bottle) it had quite a vinegar tang. What was in the sample tube, covered with clingfilm and kept alongside the vat was already quite pleasantly drinkable but the bottled stuff is possibly less so. The gravity reduction may be partly down to the production of vinegar rather than alcohol!
I suspect the relatively slow fermentation with natural yeasts didn’t produce enough of a CO2 blanket to protect it from the relatively large headroom of air in the vat, which was only about 20% full. It won’t be wasted as even cider vinegar has its uses, for example as an acidifying, tenderising agent in cooking. However, on my next batch, I will aim to use a smaller fermenting vessel just in case this was the cause.