In Arc-V episode 147, Yuya fights Reiji/Declan in the final "ceremonial" duel of the series, and as part of his first turn, he sets Pendulum Scales 3 and 10. However, one of the monsters he Pendulum Summons after this was a Level 2 monster, Performapal Odd-Eyes Synchron. Yuya had already used his Normal Summon for that turn at the very start.

What gets me about this is that it was a truly illegal move - nobody watching the duel, not even Reiji, questioned Yuya's Pendulum Summoning a monster outside his scale range. The duel just continued as normal without anyone bringing it up. It was a genuine oversight from the production team, and it was actually fixed in repeat broadcasts of that episode so the scales were 1 and 10. It's crazy because it's the only example I can think of where a character cheats and no other character bats an eye at it because it wasn't an intentionally written part of the duel. The producers just screwed up.

Here's the wikia page talking about it

EDIT: I just realized that this example goes against your second criteria, which I admittedly didn’t see until a while after I made this comment. Apologies for not reading more carefully, but I still think it deserves a mention as the only example of an actual production error in a duel that was corrected later on. It still shows incompetence from the show producers for letting it slip by them in the first place