Can anyone tell me why? I used to be a nazi with this but I have done it a couple of times with no apparent ill effects.
Well, I did it pretty much for the entire time DD was formula fed (from birth to around one year), and she never, ever had a problem with feeding and at 17 months of age has never been sick (maybe I was feeding her a bunch of bacteria that strengthened her immune system?).
I claim pure ignorance on that one, I never knew it couldn't be reheated... I had a system going where if she didn't finish a whole bottle, I'd stick it back in the fridge and top it up with fresh formula from a new bottle, then I'd top the second bottle up with fresh formula out of a third bottle, and so on... I just couldn't stand the thought of throwing out what I figured was perfectly good (not to mention EXPENSIVE!!) formula, and like I said, she never, ever got ill or anything like that. I mean, I never left formula sitting out for ages, she usually sucked a bottle dry in about fifteen minutes so if she quit before then (ie was full) I'd just whack it back in the fridge and use it later. I do the same thing now with her cow's milk, sometimes I heat a bottle up and she isn't hungry so I just pop it back and use it when she's ready for her next bottle.

Now that I know, however, I won't repeat the same mistake with the next one...