Why does the same brand of beer taste different out of a can vs. out of a bottle or keg?
Turns out, it's because of formaldehyde.
A small amount of formaldehyde used to be put in an emulsion used to lubricate the cans on the canning machines. The formaldehyde kept bacteria from breaking down the oil component of the lubricant. The emulsion got washed off before the liquid is poured in, but enough is left behind to affect the taste (not enough to be toxic).
They haven't used formaldehyde in decades, but because everyone was used to the flavor of their favorite beverage (this goes for any drink in a can), all new and improved lubricants since have been made to taste just like the original formaldehyde mixtures. Mmm, yummy!
http://www.nd.edu/~ndmag/can1su00.htm