I was just watching "Heart Of Glory", and it has an odd effect in it. Just after they Klingons have broken out of the cell and killed the security guard, then the second security guy comes around the corner and gets into a phaser fight with the one Klingon, and the officer fires twice, before his third shot finally kills the Klingon, but then Koras comes out and kills the security officer. The odd effect is when Koras' phaser beam hits the guard, we see the beam dissipate in round waves from the guy, and he dies. When the guard shot the first Klingon, we saw the dissipating lines, but it looked like that was because of the Klingon armor dispersed the energy, yet, as far as we know, no one on the Enterprise has body armor that dissipates energy away from the person. So we should've just seen an orange area where the phaser burned through the body.
With Arsenal, that's not the only episode that has a bad looking planet set. Check out Troi/Riker's holo deck scene in "Haven", or Q's planet in "Hide and Q". Or Omicron Theta in "Datalore".
But in "Arsenal" I don't get why the orbital shots of the planet are green, but the sky on he planet is blue. It would've been nice if they had changed the color of the planet, or like in "Bread and Circuses", added some clouds to the sky.