Yeah. And sometimes it is a different color, size, or has what looks like ink sqirting over it. I guess the ink is supposed to be scary ominous clouds.
This isn't a slipup, it is just really bad effects. They wanted a moon in the shot but there wasnt one so they digitally editied it in, but with the low budget they had and the time the movie was made they couldnt blend it into the shot very well. I noticed it the first time watching too but I knew it was no slip-up.
this was simply a matter of bad editing way back when. They needed one, so the spliced in a moon. If you listen to the commentary on the dvd they point it out themselves(I beleive that was in the sam commentary not the bruce).