Well, i just heard about not using growlers today in this thread, but i haven't had any problems yet. I have 4 growlers with festabrew wheat and 2 with brewhouse blonde ale and i haven't had any explode. I have the black plastic caps with the clear plastic cone shape on the inside.
When i opened the ones i have drank it was a little hard to get the cap off, and i did notice that there wasn't as much of a fizz sound as bottles of the same batch. I also noticed that the beer was a little less carbed than the bottles.
Is it possible that this type of cap actually let excess pressure out? Personally i can't see the glass breaking before the plastic cap gave out, but a metal cap would be a different story.