I have never heard of this happening and it seems unlikely. It would have been good to see a picture bt the only way I see this happening is to load wells in the middle of a gel and run the samples in the wrong direction then misread the dna band as rna but then rna is usually a diffuse band and dna more of a compact band unless it is degraded. The usual effect of high voltage is to melt the gel but even in the melting process I cannot see why small molecules that move easily through a gel should somehow appear large

I have never heard of this happening and it seems unlikely. It would have been good to see a picture bt the only way I see this happening is to load wells in the middle of a gel and run the samples in the wrong direction then misread the dna band as rna but then rna is usually a diffuse band and dna more of a compact band unless it is degraded. The usual effect of high voltage is to melt the gel but even in the melting process I cannot see why small molecules that move easily through a gel should somehow appear large

Are you sure that that is what has happened? More likely you've applied the current in the wrong direction as Paul Rutland says, or you simply inverted the gel or gel picture.

Add the picture of your gel and give us more details about the voltage that you have applied and your samples to be able to help you.

Picture absolutely needed here.

Of course supercoiled plasmid DNA runs faster in agarose than the same size linear or relaxed DNA, so there is a condition where higher molecular weight will travel further than a somewhat lower molecular weight.

Yes, The impact on the movement of the DNA and RNA