Is this a modern episode produced for high definition television? If it's an older episode produced for standard definition NTSC televisions (Japan used NTSC just like North America did), that gap would probably have been in the "overscan" area which wouldn't have been visible on most televisions.It's a very clean show in terms of animation, but this is kinda sloppy put together at the same time. This is like the fourth or fifth time in seven episodes I've noticed the cels have been poorly placed in frame.
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I haven't seen an animation cel so sloppily done since Souryo to Majiwaru Shikiyoku no Yoru ni... :/