Harmy said:

No, that's not really it. It's hard to tell from YouTube but look for specs of dirt - Since the CG shot that replaced this one in the SE used the same background, I replaced the parts of the background that were usable from the SE and then I cleaned up the the remainder of GOUT footage frame by frame in photoshop.

That was going to be my guess. I always found that interesting that they rendered the CG on the same plate as the original effects.

hairy_hen said:

The melted bars thing really baffles me. Presumably that was an actual on-set explosion and not a special effect added in post-production, right? So why did somebody take it upon themselves to think it was not good enough? Is this a case of "Reality Is Unrealistic"? (as they say on TVTropes) Somebody thought what was actually photographed didn't measure up to their conception of what it ought to look like, even though that's what it actually did look like? And for just a couple of frames nobody would notice on their own? What the heck is that about?

I don't think there was an on-set explosion that blew out the bars. There were probably two sets of grates, one full and one cut open. They show the full grate, then replace it with the broken one and blow up something behind it. It's a jump cut, just a pretty well covered up one.