when do leftovers go bad
Double-dipping will make food go bad faster because it transfers saliva into the food, which promotes bacterial growth. One study even found that three to six double dips transferred about 10,000 bacteria from an eater's mouth to the dip sample. Putting leftovers away in a big clump may also promote food spoilage.
If you leave leftovers or any food out too long, it will speed spoilage. Two hours is generally the max they should be left out, though the sooner you refrigerate them the better (remember to factor in the time it takes to drive back from the grocery store, and the time it takes you to unpack your groceries).