Slap scratches, also called loop scratches, are caused by threading the projector with the loops to big or too small. In many cases, the damage will be imperceptible after one or two passes, but reveal itself after several runs on the same projector using the same threading pattern, and appear as small vertical tic marks, sometimes in clusters.
A common myth about loop sizes and slap scratches is that a projectionist will be able to tell if a print is scratching simply by advancing the projector by hand with film threaded, and observing the upper and lower loop. However, once the motor comes up to speed, the loop begins to oscillate quite a bit more. Only by running the motor at full speed can you observe whether or not the loops are hitting the inside of the projector.