Board Thread:Questions and Answers/@comment-28194879-20160519003219/@comment-5774380-20160519130910

You actually mentioned one of them: One Piece. They have had no less than 12 filler arcs as you can see here. They stopped doing that some time ago, but now they insert filler in each episode to compensate; which is even worse.

Another legendary example is Dragon Ball Z with the filler Garlic Jr. Saga (which creates a huge plot hole as it is linked to a movie-only event incompatible with both the manga and anime) and those eternal five minutes of the final fight between Frieza and Goku. But they had a lot more of filler, and that's why they even tried to create a second anime adaptation more faithful to the manga in the form of Dragon Ball Kai.

But these aren't isolated cases. All animes have filler in some degree, but the ones that are aired uninterruptedly require higher quantities of it to avoid catching up to the story of the manga. They explain this situation in more detail here.