Warning: spent 3 weeks chasing a light leak that was just my changing bag zipper
Replaced the seals on my Pentax K1000 twice, taped every seam, and burned through 4 rolls before I realized the white streaks were coming from a pinhole in the zipper of my $20 changing bag, so has anyone else blamed a camera for a bag problem?
The part that gets me is how a changing bag leak looks exactly like a camera leak, so your brain goes straight to the expensive thing first. There's actually a trick worth knowing: load a roll of cheap film, pull the leader out, and leave it sitting in the bag in direct sun for an hour before you ever run it through the camera. If the film fogs, the bag is the problem, and you just saved yourself a full tear down. Zippers fail in a way that is almost invisible too, since the pinhole is usually right where the pull sits and the fabric folds over it. Also worth checking the seams where the arm holes meet the body, because those rub thin way before the zipper does.