Don't blame me, I'm an interpreter.

This time it's C3PO's leg. Actually, in the 1977 Star Wars movie, the first to be produced, he's the very first character you see on screen and this effect is present right from the start.

In the films seen today, his right leg from the foot to the knee is silver, with the rest of his body being gold. Most people just remember him being gold all over and are quite surprised to find these are alternate memories when they see the silver leg since once it's pointed out to them.

This one is also backed up by the merchandise such as toys and posters, which also feature the silver leg, as well as related "guest" appearances such as at the Oscars. All show a silver leg, yet few people remember it.

The "official" explanation is that people didn't notice because it always reflected what was around it, such as the other leg or the desert, and since no explicit reference was made to it (it was intended to show he was battle scarred), it passed by largely unnoticed.

Anthony Daniels confirmed it

In 2015 the actor who played C3PO confirmed the silver leg was there all along:

Even the stills photographer, John Jay, came up to me one day and said, 'Why are you wearing a silver leg today.' Now, he was the stills photographer, and he hadn't noticed.

Here's a shot from the set where Harrison Ford is helping Anthony get up - apparently the costume didn't allow for much free movement, and he couldn't even sit down in it!

It's amazing that m,any of those who remember it being all gold still have that memory even though all this evidence of it always being silver is present. They acknowledge it too, it's as if there is some kind of dual memory at play which is impossible to explain.

Here's YouTuber scarabperformance's take on this: