I noticed the other evening that I'd forgotten to buy parsley - parsley! What a silly thing if you don't need it for a recipe. Yet I realized there are a few simple items I have to have on hand to make dinner out of nothing:
parsley/other fresh herbs
red pepper flakes
peas or spinach
Of course there are many good and useful pantry staples, but I've found that I'm actually a little anxious if I don't have all of the above on hand.