Two things I will ALWAYS at least try to correct (especially from incorrect salespeople) are bad info about firearms and cameras. No need to be rude about it, start off as oldgringo mentions above. Or wait til salesman goes elsewhere for a minute and then casually/quietly offer suggestion to customer.
With cameras I've kinda become an old coot about it though. Everytime I hear some kid at Best Buy say the equivalent of "this camera is better beacause it has 10 megapixels and the other only has 8" I wait until he turns away and then politely tell the customers
"Hi. I couldn't help overhearing what he said. I was a semi-professional photographer for 4 years. The naked human eye cannot detect the difference between 8 mp and 10 mp. Furthermore, I've printed and sold for hundreds of dollars images as large as 3 foot x 4 foot from a 4-megapixel camera. Any amount of pixels available in any camera in this store is more than you'll ever use or need. Go with the features you like best and don't worry one bit about megapixels. It's a meaningless camera sales gimmick."
With guns I'm more deferential, but would correct an obvious mistatement, especially if it is related to safety like "never cleaning a Glock"
