The droid C-3PO has been in all the Star Wars movies, but even though he has always been played by actor Anthony Daniels, the character doesn't always look exactly the same. He opened the whole saga in A New Hope, along with R2D2. Then in the prequels, we find that he was built from spare parts by the child Anakin Skywalker. That seemed a disappointment, an obvious clue that plot points would be shoehorned into the story as an excuse to bring back familiar characters.
But back to C-3PO's appearance. A casual fan may have missed it, but the droid had a mismatched shin in the original trilogy. His left shin was silver, while the rest of his body was gold-colored. This was made clear by the appearance of the action figure, but wasn't so obvious in the first movie, because the silver shin reflected the gold color of the other leg. Why was he mismatched? We know his memory was wiped at the end of the prequel trilogy, and he served Bail Organa afterward, but no explanation for the shin was ever given in the movies.
According to Anthony Daniels, there was supposed to be an explanation eventually, but it just never happened. For observant fans, there is plenty of speculation to chew on, which you can read about at CBR.com.