Hope it's not an infection and that the flavor mellows with time. I made a pumpkin beer a few years ago and it's never going to mellow. I consider it a failed experiment. A friend liked it enough to take some off my hands but the rest is going down the drain as soon as I need to use the keg. And the keg will probably need to be taken apart and the o-rings soaked or replaced, too. If it turns out it is an infection, good cleaning and sanitation of everything on the cold side (post-boil) that touches the wort usually prevents it from happening again. That and very quickly cooling the wort to pitching temperature so that exposure to air is minimized. I probably should do my cooling inside the house once flameout has occurred. There is actually a test kit I saw online somewhere to test whether your beer is infected or not. This is the product (just looked and found it) but the supplier is in the US: https://www.morebeer.com/products/fastorange-ready-tubes-2-5ml-swabs.html But other than maybe telling you that's what happened, it doesn't do anything to fix things for you. I think the test kit is almost pointless unless you're trying to troubleshoot where an infection came from (like the spigot on a bottling bucket, bottle filler, etc.) or just want to confirm a beer is infected. If it still looks fine to the eyes in a few months with no ropy looking bacteria slime in the beer, you probably are right that you just have a bad combination of ingredients. I had one batch that I ordered what I thought were the same ingredients that was in a kit from a homebrewing supplier, but it turned out that in their kits they must have used a different brand of dried malt extract. It was dark extract, same as called for in the recipe, but it did not turn out anywhere near the same taste as the kit. No infection was present, just one of those things. They don't label their recipe nor their dried extract as to which brand it was. I thought I was saving a few dollars by making up my own kit but it turned out to be a beer that wasn't all that great. Drinkable, but not refreshing. More for medicinal purposes, I guess, after a bad day at work. The place makes some great extract with grains kits, though. I've moved on to all grain and because of time and storage considerations am going back to extract with grains again. I hope to brew again this year after a break of a few years since my last brew, probably this spring (your autumn).