> Please tell us how one can screw up hooking up a toilet so badly as to create a cross connection

Sure: unless you understand potable water and drainage systems, proper appurtenances, proper methods and materials I don't want you touching my drinking water supply. It's the same reason you don't want me typing random characters into your code. I'm not a coder, you're not a plumber.

And to be clear I used domestic water as an example, you could just as easily mess up the sanitary connection and have raw sewage seeping into the building.

It takes 8 years of working experience before you are even allowed to take a Master Plumbing test... and we aren't stupid people. There is A lot know.