If you've got a system where you can tweak the ram timings and/or voltages (most desktops will let you do at least some of this), it's pretty easy to induce failures, if you haven't run into bad ram naturally.

I've run enough systems with ECC ram to say the vast majority of ram works and will continue to work, but over time, a small amount will fail, again the vast majority is a single bit in the whole chip, but some will do multiple bits near each other (uncorrectable ecc error usually halts the system), or man single bit errors all over (many machine check errors per second makes the system go so slow, halting would be better).