My suspicion is that real {ale,cider,perry,comminuted-badger-juice} makers don't use some sort of heavy-duty filtering process that large-volume commercial breweries use, and so there are more weird toxins left in the result. An acquaintance of mine discovered that 1cc of isopropanol gives a hangover comparable to an entire beer festival ...

The test would be to drink the same volume of the same stuff at a beer festival and a normal pub, and this rather defeats the point of a beer festival.

*nods* Trace toxins other than the alcohol itself were certainly the obvious explanation that occurred to me. I hadn't quite got as far as thinking about how those toxins got there, but certainly I'd have been after some sort of explanation that was common to both the beer and the cider at beer festivals, so your filtering explanation sounds plausible on that score.

You'd like the ciders they sell in Finland then - they're all sweet. Kopparberg, Fizz, Upcider, Recorderlig. And they make some interesting varieties. I especially like the lime and wild strawberry one, and the starfruit and gooseberry one. The coffee cider, on the other hand, is disgusting.