[Joe Nation]

I was in my car the other day, with my ME Super sitting on the seat next to me. I went to put it under my seat as I was getting out and didn't want it on show, and the edge of the film door caught on my shirt and popped open. Needless to say the four or five shots I'd already taken are ruined, but I was wondering if there is a problem with the door latch or if it's just a minor design flaw.

[Nesster]

It should not pop open just like that... unless you pulled up the rewind knob at the same time. When you're done with your film test the back to see if you can pop it - that shoud not be happening, if everything is lined up right.

Another possibility is the usual 'user error' - I'm remembering many 35mm cameras have a sort of two stage click when closing the back, and if you don't get to the full latch the back may pop.

[Sluggo]

I have an ME Super that used to do that. Turned out the pressure plate in the film door was installed backwards so the door never closed properly. It was light tight when closed, but the latch never quite fully engaged, and the door would pop open on the slightest provocation. The fix was to turn the pressure plate around the right way, and un-bend the door latch which had deformed slightly from the pressure. It's been fine ever since.

That wasn't Pentax's fault. Pretty sure I screwed it up during an earlier repair; I had once removed the pressure plate while reattaching a broken film minder frame. Must have flipped it around during reassembly, having failed to notice that it wasn't quite symmetrical with respect to the mount points.

[MysteryOnion]

Another possible cause is the door seal is either very gummed up and stick which could cause the door latch to gum up and not latch.
Sometimes the seal is not foam, instead, the previous owner had given a little homey touch and put yarn or cord in place of the correct material which could also prevent the door from find the latch.