I can think of two potential problems:

1. Density of running buffer would be greater than that of the sample buffer. As you fill up the wells with your sample it will not sink to the bottom of the well.

2. You are changing the ionic strength of the running buffer, which would have an affect on the coulombic force felt by the individual components you are trying to separate by applying an electric field across the gel.

Cheers,

John

proper electrolytes will be disproportional to cause smear like pattern on SDS PAGE band with improper resolution