Horizontal gels are cooled by the cooling plate underneath.
At the lateral edges a horizontal gel is additionally cooled from the side:
All horizontal gels are a bit warmer in the middle and a bit colder at their lateral ends.
If overheating occurs the electrophoretical bands are not anymore migrating in a straight line. They are bend because running quicker in the warmer mid part of the gel.
If you are looking at the gel, cathodic side up, it seems that the bands are smiling at you: The “Smiling Effect”!
To overcome this:
a Look after your cooling! Swiched on? Right temperature? Flow rate through the instrument(s) is about 500 ml/10 sec
b Lower the mA and the W