Browse the op code list with a creative mind. The days of erratic single bit RAM failures are thankfully gone. Once DRAMs were subject to cosmic ray and even alpha particle problems, so designers came up with exhaustive tests that insured no bit could interact with any other bit within each chip. New packaging materials cured these problems once the chip vendors discovered that the plastic material used to encapsulate the silicon was one of the biggest sources of alpha particles. Now it seems most RAM failures stem from good old-fashioned electrical and logic problems.
RAMs fail outright, just as any other part does.
Decoders die, preventing the selection of entire RAM devices. Address and data lines may not make it to the chips, or the write signal may just peter out on its way across the board. All of these problems result in fairly massive access problems. An effective RAM test need not check every possible state of the array, as long as it tests pretty much every location. This simplification results in a huge decrease in the time a RAM test will take to run.