How likely is it that a computer miscalculates 1+1?
The probability of read error is bigger than write error, so there is a chance for bitflip on either of $1$ which might occur anywhere.
I can offer description of JVM attack which is not exactly the answer, but shows how much effort is needed to induce bitflips, tells about softerrors and a very successful attack on JVM, given good heat ;).

The probability of error grows the most related with heat dissipation problems, and making chip smaller include it's higher efficiency (performance per watt).

Anyway there is bad assumption - making chip smaller gives more errors - if this were true over time CPUs would generate noticeable amount of errors today.

Please check also this error rate study. With calculating probability of error there is a problem - this is hardware dependent and the most of hardware actually has errors, but they get corrected at several levels, so they do not occur for user, and with invalid / broken piece machine crashes are to often to perform test.