In one case, some of the smaller fragments may have run off the end of the gel. This frequently occurs if the gel is run too long. A second possible source of error is that the gel was not dense enough and therefore was unable to resolve fragments close in size.
In one instance, some of the smaller fragments may have run off the edge of the gel due to its length. This can be caused by a lack of density in the gel, which prevents it from resolving small fragments.