So, I wasn't trying to do a chemistry experiment this evening; it was entirely unintentional!
I was mixing up a batch of fertilizer for my plants (no nitrates involved, so I wasn't worried about potentially dangerous reactions). The main goal was to add some sulfur (which over time lowers the pH) and potassium (in the form of potassium sulfate); to balance it out there also were lesser amounts of calcium superphosphate and magnesium sulfate, a little iron sulfate, and tiny amounts of trace elements (boric acid and some simple compounds of manganese, copper, zinc, and molybdenum)
The potassium sulfate tends to come in big chunks because it's hygroscopic, and I know that hot water helps dissolve it (and that the sulfur doesn't dissolve in water, even hot water), so I heated the mixture (in water) in the microwave, then stirred with a plastic utensil. I noticed a surprising amount of offensive-smelling offgassing coming off of it. So I put in my analog pH meter, and it pegged instantly, hard (its limit is 3,5, so it must have been much lower). Even after adding some calcium oxide (which instantly fizzed and started gelling), the pH was still pegging hard. And after 1 dilution (maybe 7:1?), it was still pegging, and only after the second dilution did it get stop pegging (but still showed strongly acidic). There was heavy brown discoloring in the bottom of the plastic container, and it looked like the sulfur might have been reacting as well.
I assume I created sulfuric acid (and/or possibly phosphoric?) somehow, but I'm not sure what the reaction was! I wasn't expecting this at all. Normally in horticulture, sulfur takes ages to acidify soil, as it's decomposed by bacteria. I know that combusted sulfur yields sulfur dioxide, which - oxidized over a vanadium pentoxide (or other) catalyst - yields sulfur trioxide, and thus (with water) sulfuric acid... but that's clearly not what happened here. Any clue what reaction we're looking at?