What the warning (incorrectly) says: When they get old cake mixes grow spores that can kill you, which is why it’s so important to throw out your expired cake mix — it is toxic.
What the warning leaves out: Cake mix does not develop spores over time, but cake mix that isn’t sealed (like lots of other things left unsealed) can get moldy if exposed to mold spores. People who are allergic to mold can have serious reactions in the presence of mold. But old cake mix is itself not toxic.
Very important thing: If you are allergic to mold, you can have a severe reaction to eating anything with mold in it. And if you do not seal your cake mix, your cake mix can get mold in it.
Therefore, if you’re allergic to mold, you should take precautions and make sure your cake mix doesn’t get mold in it. Usually it comes in a sealed plastic bag anyway. Pancake mix might be a bit trickier–maybe mold-sensitive people could put it in a sealable plastic or glass canister immediately, or simply buy small packages and make and eat it quickly.
More information: Snopes.com.