Whoops, half this negative is overexposed

Ok, on two seperate occasions, two types of film (one color, one Ilford HP5) two different cameras, and two different students have had negatives that were neatly divided into halves vertically where one half was properly exposed and one half overexposed. I ended up teaching photography at the local high school after a 20 year vacation from the darkroom and I can't come up with a reason for this occurance. Both students have had perfect films from these same cameras. I think it may have to do with improper flash use but can't find a reference and have never had this problem personally. Any input would be appreciated. Thanks

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