These things do happen with a reasonable amount of frequency. I used to work at a division of a major memory manufacturer that dealt with writing tests to find these DIMMs that exhibited these sorts of failures - the semiconductor industry calls them "variable retention transfers". (Aside: numerous PhDs in the field of semiconductor physics have built prosperous careers trying to understand why these soft failures happen. Short answer: we have some theories, but we don't really know.) It was provably worth millions of dollars to be able to screen for this sort of phenomenon, because a Google or an Apple or an IBM would return a whole manufacturing lot of your bleeding edge, high-margin DIMMs if they found one bit error in one chip of one lot. Each lot was shipping for millions and millions of dollars.