Stuff can sit in the back of your pantry for way too long. And while it can be hard to know if you should toss or use certain items, when it comes to that box of pancake mix, you should toss it (especially if it's open). Technically, it can kill you, but only if you're allergic to mold.
A study published in the American Journal of Forensic Medical Pathology detailed the death of a 19-year-old male who ate pancakes from an opened pancake mix that was two years old. After eating two pancakes, he went into anaphylaxis (unable to breathe) and died. Later, it was revealed that the pancake mix contained high amounts of different types of mold (including penicillin) that he had severe allergies to.
There have been other cases, too. Another study published in the Annals of Allergy, Asthma & Immunology found that a 52-year-old women went into severe anaphylaxis "on 2 separate occasions after ingesting and inhaling cooking vapors from a commercial pancake mix contaminated with the house dust mite Dermatophagoides farinae." Thankfully, she survived, but how gross is that?
If you don't have a severe mold or mite allergy, you might be OK to still eat it, but it's probably not worth the risk. According to the US Department of Health and Human Services' FoodKeeper App, which allows you to check how long you can keep certain foods, pancake and biscuit mixes stored in the pantry should be used within nine months.
So, if you're unsure about how long that half-used box of pancake mix has been there, just toss it and make a quick run to the grocery, or whip up your own mix from scratch to avoid a potentially fatal situation.