It may seem obvious but when you attach the film to the spool make sure it is straight and not canted one way or the other. If its a little off it will rub against one side of the spool creating friction as it feeds into the camera. Not a huge deal with manual wind cameras but with modern slrs that have auto rewind the friction could cause it to rewind midroll due to the sensor thinking the roll is finished. I found this out the hard way when I first started bulk loading and had this scenario happen a couple times in my Canon.