Check this out on Snopes & you'll see that it can sometimes be true.

http://www.snopes.com/medical/toxins/pancake.asp

Check the date on your pancake mix-This is from a real Dear Abby
article...

DEAR ABBY:
I recently made a batch of pancakes for my healthy 14 yr old son, using
a mix that was in our pantry. He said they tasted "funny," but ate them
anyway. About 10 minutes later, he began having difficulty breathing &
his lips began turning purple. I gave him his allergy pill, had him sit
on the sofa & told him to relax. He was wheezing when breathing. My
husband, a volunteer firefighter & EMT, heated up some water & had our
son lean over the water so the steam could clear his chest & sinuses.
Soon, his breathing became more regular & his lips returned to a more
normal color. We checked the date on the box of pancake mix & found it
was very outdated. As a reference librarian at an academic institution,
I have the ability to search through many research databases. I found
an article the next day that mentioned a 19 yr old male DYING after
eating pancakes made with outdated mix. The mold that forms in old
pancake mix can be toxic! When we told our friends about my son's close
call, we were surprised at the number of people who said they should
check their own pancake mix since they don't use it often, or they had
purchased it some time ago. With so many people shopping at
warehouse-type stores & buying large sizes of pancake mix, I hope your
readers will take the time to check the expiration date on their boxes.
Also, beware of outdated cake, brownie & cookie mixes.
Sharon