This can happen if the cups have any scratches in the glass deep enough to create an expansion area. I was my cups religiously and do a thorough check for scratches or minute cracks. I've had more than a couple do that and they were relatively new. I've never had one pop that was older Pyrex. I use a lot of 4 ounce pyrex custard cups, some of which are as old as I am or older and none of them have ever blown, but the newest have..