Where was this screw supposed to be?

Why is it that every time you take something apart, and then put it back together again, there is always an extra piece?

Frequently a washer, often a screw, sometimes a piece of metal or plastic that honestly doesn't look like it was part of the thing you just took apart. There is always something left over.

But the most confusing thing is that the item that was was taken apart, and then put back together, works again. So did it really need the left over bit? Was that left over part really necessary? Did it really come from the item in the first place?

If there are parallel dimensions, is there one where something is lost every time an item is taken apart and then put back together? Is this left over screw from that dimension?

Is this the key to traveling to another dimension? Become an integral part of a mechanism, wait for it to break down, and then escape to a new world when someone takes it apart to fix it? Actually, no, that would only be from the other dimension to ours. It's our dimension where the extra pieces appear. So is this where aliens come from? Did someone in a parallel dimension already have this idea, and they managed to jump to ours using this scheme?

Is this an alien screw from another dimension?