Feed can spoil if it's left kicken around in the back of the store before they put it out for sale. I've known people who've had this happen to we only buy a bag at a time and check it over carefully just to be safe. But we also only have chicks so a 50lb bag lasted us 2 months.
I had just bought it that morning and it had looked fine. Wasn't fun. Sent some to pen state to get tested to find out for sure.