Too fast migration of the dyes, gel is sweating and later burning:
The dyes of the samples and the gel should migrate around 0.5 mm per minute, not more!
1 Too much currant, too high mA value:
a Power supply is not programmed correctly
b A part of the gel runs with the conditions of a whole gel
2 The cooling device is not running correctly, or it is not pumping really through the cooling plate