It does look a bit like that but no, it is just the result of normalising each row of a spectrum which is slightly tilted. Here is the whole “corrected” flat. The vertical gain correction algorithm only works correctly if the dispersion is exactly along the row direction.
This is a severe obvious case but some variations along the slit are much more subtle. For example these in a highly stretched LHIRES flat image from last night are down at the 1% level and are in the region where I take spectra. They are not anything to be too concerned about as an uncorrected flat should work ok to divide them out but not if they are tilted and I apply the vertical gain correction
