Spongebob Squarepants - The Incredible Magical Color Changing Rope
This is really obvious. It's almost impossible to miss. I noticed it the first time I saw this episode and so did everyone else who I showed this episode to. (I have the episode on DVD.)
At the beginning of the episode Squidward, SpongeBob, and Patrick are climbing a rope to see what's at the top of the anchor. At one point all three characters stop climbing the rope, (to say something), and are centered in the middle of the screen. You'll noticed the rope is a very dark black. As soon as the characters stop talking and start climbing again, the rope magically changes to a very light brown. It's very obvious because the rope is centered on the screen the whole time. It doesn't change between scenes either, it changes in the same scene!