snopes.com: Pancake Mix and Allergic ReactionUnlike what the e-mailed caution would have folks believe, the foodstuff has to be exposed to its spores. Pancake mix cocooned in an unbleached wax paper, plastic, or a foil pouch within its outer packaging wouldn't have this contact and should still be safe no matter how old it gets. However, mix sold unpouched in cardboard boxes or paper sacks would likely be at risk even if the box or sack hadn't previously been opened, because such packaging would not necessarily keep dampness out, and mold thrives in damp environments.
What does all this mean? If you don't have a mold allergy, you needn't fear your pancake mix; if you do have such a sensitivity, you shouldn't keep your flapjack makings around a few years after opening the box or pouch it came in. It's not worth dying over 50 cents worth of pancake mix, so when in doubt, throw it out.