Memtest found a error

Both at once. Just let it running through the night.

Well, i couldn't reproduce it, so i guess its fine.(?)

I ran the test just to be safe, the first result managed to scare me.
 
You should run one at a time so you can tell which one is at fault, but as it hasn't happened again, its up to you whether you risk them working fine from now on or get replacements

I have never run memtest as i have never had memory errors. I guess it makes sense to run it on new ram but I won't be buying any for a few years.