Sticking your nose in a carboy can be bad...
But sticking your head into a keezer fermenter, the day after your yeast takes off, is deadly.
I got a burning whiff and nearly passed out into my keezer. I almost ended up being the adult CO2 version of the "danger of toddler drowning" warning label you see on the side of 5 gallon buckets.
I've inhaled quite a few gasses in my lifetime and CO2 gets the cigar for being the most painful of 'em all. Even if you hold your breath, carbon dioxide will cause a burning sensation through the moisture in your eyes, and nose.
EDIT: I can't help but wonder how many brewers are going to take a big huff of co2, simply because we told them not to. lol.