Friend of mine recently brewed a sour cherry wheat. Had it a month ago at his house, and it was delicious. I'm not a big wheat fan, but the cherry melded with it pretty well and created a beer of which I'd have had another.
We met for LHBC on Wednesday. He brought a growler he'd filled a couple weeks ago. When he opened it, GEYSER!
I had a taste and it was clearly infected. Very thin, almost no flavor of either the wheat or the cherries. A dumper.
It was my theory that the growler wasn't cleaned properly and that's what caused the infection. Unfortunately, he checked the keg it came from, and it's infected too. A dumper. So much for my theory.
So--when people say drink it fast, yes. And figure out where your cleaning or sanitizing or controlling outside infection agents is failing.