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).

As mentioned below it can deteriorate over time. A while ago I had a RAM stick fail. I noticed because GCC started segfaulting randomly and I haven't upgraded GCC recently. Sure enough a memtest revealed the fault. I think it didn't have enough cooling.

I only purchased RAM with heatsinks since then, and I didn't have such problems anymore (also my latest case has much better airflow)

I unplugged the wrong power cable once, and thereafter the RAID controller on that machine would report degradation after the system was running for a while. Finally ran a RAM error check, one bit in most of a stick was flipped. The last stick. So once the machine was all spooled up for a while, I’d get errors.

This is also how I learned 3ware was using RAM instead of its own memory. What is he point of hardware RAID if main memory can screw it up?

I have also heard of people swapping hardware on a defective machine only to learn that the power supply was bad, sending noise or brown power to the system. Swap in a new power supply and the components start to behave themselves.