Random memory errors also occur with perfectly good RAM. Cosmic rays flip bits. I recall that the rate is about 1 bit per gigabyte per month.

To prevent it from impacting your machine, you use ECC memory. Servers typically require this type of memory (which uses 36 bit words rather than 32) and some desktops will use it, but you can't mix ECC with non-ECC memory. Also most ECC memory that you find is server memory, which is "registered" or "buffered," meaning that there is an output latch for the data, and I have never seen a desktop that could take it.