For me it's someone opens a jar or other food container with a seal -- like the little layer of plastic of foil between the lid and the food -- and instead of removing the seal completely, they just peel it back enough to get at the food. When they're done the put the cap back on, leaving the next person to deal with the flap of seal clinging uselessly to the top of the container. This drives utterly bonkers, grossly out of proportion with the two seconds it requires for me to finish the job. Fortunately, I am well aware this is a me problem, not a them problem.
> But if the seal is laid back down it can act as an additional barrier to air, potentially making the food last longer.
More than likely, you've touched the inside of the seal, and by laying it back down you've made accidental contamination of the jar's contents very easy.
You might as well double dip (which is my pet peeve, along with variations like reusing the knife you're spreading mayo with to get more mayo out of the jar).
