L.M.
I would say no big deal because cereal has a long shelf life and chances are you'll eat the entire container long before it goes bad.
Keep in mind the "expiration date" on cereal doesn't mean it's expired and needs to be thrown out. The "expiration date" is the recommended use by date. Add that to the fact that keeping it in an airtight container also keeps it fresh much longer, especially in this humid weather.