What would cause a memory failure?
Once the memory is installed and it gets along with the computer, it is usually maintenance free.
I've never seen a module go bad on its own without being provoked by something such as overclocking, mishandling, etc.
On my old office work computer, it's been working fine for years with 3 x 128MB SDRAM PC100 modules. It memtest86'ed fine at the time.
The computer was acting up, so I ran Windows Memory test and found one of the modules is consistently giving errors. I moved the module to a different box and tested in that box. Same failure pattern. fffffff shows up as ffffbf or something every time.
How common is a memory failure of this type and what causes it?