But it isnt really 20% sanitizer is it?
I mean lets say he added 20 grams of sanitizer (at recommended dosage though it was probably less), which is half sodium percarbonate and half sodium sulfate. The percarbonate breaks down into carbonate and peroxide, and the peroxide decomposes into O2 and water. Since it is disolved, you are left with nothing but sodium ions, sulfate ions and carbonate ions. Nothing more.
Sulphate is regularly added in the form of CaSO4 (gypsum) to increase hop bitterness, carbonate is omnipresent in groundwater.
Sodium levels vary, but if you use softened water which started quite hard, you could easily have these levels of sodium as well, depending of course what levels the OPs water had when he started. Heck gatorade has nearly 1000 milligrams of sodium per gallon. A single dose of eno is like 850 milligrams.
Hey, I'm no expert but this looks a lot more like a mineral addition then anything else. It is certainly not ideal, but what could we learn? We could learn not to fear a little percarbonate residue, if a completely over the top addition didn't make undrinkable beer.
We could also learn how to simulate burton-on-trent with a little sanitizer! (okay, sorry that was a joke)
edited to add: I don't want to create the illusion that I have the first idea what I am talking about here, this is merely my impression based on other things that I have read on these here interwebs and a high school diploma. There could very well be something more going on than my simplistic approach indicates, that someone with higher education and more intelligence than I have access to would understand.
edited a second time: I looked up the prevalence of sodium in drinking water, and found that the upper limits in public sources tends to top out around 250 mg/litre, and that over 200 mg/litre tends to be objectionable. Of course this is water where off tastes are more easily noticed. Assuming 20 grams of sanitizer in 5 gallons of beer, and sodium being 44 percent of the weight of the sanitizer, that would leave you a little over 460 mg/litre. So it sounds like the beer might taste a little salty?