Unknown effect is tarnishing my work

how can i get rid of that one bit of the wall looking so much brighter that the rest of it? I've seen this happen before but in this case its quite bad and it almost looks like another rock texture is being used, in fact when i first saw it i was convinced that i had accidently used STONE6 instead of TANROCK5.

If this is what I think it is, then it is something which happens in Zdoom (not sure about other ports?), I think it's got something to do with "fake contrasting" or something, an option you can switch off in the display setting. Another way to "fix" it is to change the way the angle of the offending wall is dissected. Hope this helps.

Read all about it here.

Yeah, if you're not editing for ZDoom, a simple way to make the side use flat lighting is to offset one of the vertices (even by a single unit) so that the line doesn't align to the grid.

Yes thats worked, I should have mentioned that Im using Boom format,
problem solved, thanks.