Perculiar contamination of agarose gel combs? - (Oct/11/2013 )

My colleagues are having a strange problem with some of their gels. They suspect one or more combs are responsible and have tried to clean them to no avail. Does anyone know what might be causing the diffuse regions of fluorescence in these images?

That is unusual.  Probably not from the combs (low retention).  Check your UV tray and make sure that an undergrad didn't crack the glass and leave EtBr residue within. Do you only see this pattern when a gel is present? It isn't in all your samples, so it probably isn't in your loading dye. It seems strange...

we have this sometimes (but then only one cloud) when EtBr was not well mixed in the agarose or there was pipetted something from the "sediment" in the EtBr tube into the gel. 

another idea would be that this is smear of small fragments that comes from the lanes above the combs (if there is one, cannot see it as it's too dark)

The pattern seems to be consistent between gels - To test if it was the combs, set up a gel with no sample and run as per normal - if you see this pattern, then it is likely to be the combs, but if not, then it is something to do with either your sample or the EtBr.