i've been brewing for a few years, i've done some dumb stuff, but not really for the last year or so, untill recently.
i brewed a ten gallon all grain batch of my "death by dunkel" beer, let it sit in the primary for about 2 and a half weeks, and then kegged it, and let it sit on 12psi for about a week and a half. all normal, i've made probably over 200 gallons of this exact recipe over time.
when i took the first pull from the keg, it had a really weird taste. like i couldn't even describe it. almost rubbery. this was the first beer i had made at our new house we just bought, and i thought maybe the water was just really bad, and decided for the next brew i should use bottled water, and also check out my water profile.
well i gave a growler of it to my friend, warning him of the weird taste, and the next day he came back with some tasting notes and he said, "well it wasn't too bad, it reminded me of when i was a kid, and i would drink water out of a brand new garden hose. it kind of tasted like that."
WHAM- it hit me, that's exactly what i did. i went to menards, bought a brand new garden hose, and hooked it up, and immediately turned the hose on, and poured my mash water.
good news is, it really isn't THAT bad, and it's actually mellowed out a ton over the last three weeks.
so yeah. lessoned learned. don't do that.