The way to avoid this problem is to use an extra low adhesive artist's or masking tape to tape the holders closed. The tape is also used for writing exposure and developing information on it. With the holders taped and locked, there is no problem with accidentally exposing the film. I use zip-lock bags to guard against dust, and I also utilize the black plastic envelopes, which are supplied with photographic film and paper, to store and transport film holders in order to protect them from any possibility of inadvertent accidental exposures in the field.