Of course, normally a fully-functional computer will calculate 1+1=2. However, the physics governing the behavior of a chip is quantum mechanical. So in principle there is a certain probability that something goes wrong and we get the result 1+1=x where x≠2. In my opinion this probability should get bigger the smaller the chips are becoming.
So the question is: How likely is this event? Can one give an order of magnitude?
P.S. I'm not talking about quantum computing, just "standard" computers nowadays everyone has on their desk.