Everyone makes that mistake once. It is an understated problem in building fault tolerant storage arrays.

We buy a batch of 20 drives all at the same time, and they are all the same manufacturer, model, size etc... Possibly even from the same batch or date of manufacture. Then we put them in continuous use in the same room, at the same temperature, in the same chassis. Finally they have an almost identical amount of reads/writes.

Then we act shocked that two drives fail within a short interval of each other :)