What happens to Zoro's scabbards when he's in a fight?
I would call this a "mistake" or something similar, but I don't think that would do it justice. Not to take away from the time, effort, and skill of One Piece's artists or Oda-sensei himself, but you'll find in many of Oda's SBS columns (a section of the manga in which Oda, OP's creator, answers questions from his fans), that these kinds of questions are typically answered in a somewhat silly way. Although there's a clear effort to minimize such "mistakes," their importance is obviously not the most important thing to Oda. You'll find a favorite example of such an answer on the top answer of this thread. Not so satisfying for everyone, but if you'll excuse the speculation, I'd imagine Oda would give you an answer with the spirit of "Sometimes he doesn't like to wear them. He'll pick them up later."