Popcorn won't make you ill if it's 'expired'. The amount of heat that has to occur within the kernel to make it pop, is enough to kill any bacteria and OLD popcorn is very dry. Bacteria generally needs moisture. The way popcorn works, is inside the kernel there is moisture. As it heats, the moisture starts to steam, but there's nowhere for it to go, so it essentially explodes. So...what happens with expired popcorn is that either the oil it pops in has gone rancid (but in microwaved popcorn, it's not likely) or the popcorn kernels have dried out too much, and there isn't enough water to heat up inside the kernel to make it pop, so you'd have a lot of unpopped kernels.