Biggest mistake was not with my own beer, which made it worse. My good buddy who is newer to brewing, decided to cold crash for the first time. I gave him some basic instructions, but didn't think about telling him to pull his blow-off tube off. Turns out he had his blow-off tube in a 5 gallon bucket of iodophor mixture (don't ask me why). There was at least a half gallon to a gallon of sanitizer sucked into that batch. Needless to say that batch got dumped.
It was a batch that he and I brewed together for a shower being held at his house. Only reason the shower was at his house was because of the beer. So everyone was expecting homebrew. The batch was dumped 3 days before the shower.
Luckily I had enough in my own kegs to bring to his house for the shower.
That's a mistake I've made before and am very careful about personally. Although when I made it, it was only with a small amount of star-san where maybe a few ounces got sucked into the beer.
Yeah, I used to leave a vodka filled airlock when cold crashing until i realized I might as well just use a solid rubber or breathable silicone stopper (depending on hole size in my plastic bucket lid or carboy). It's not like I'm going to collapse the bucket!
I must admit I've cold crashed with the blowoff tube in, but those times I recall I stepped down the temp in increments and never got sucked in sanitizer.