To me, it's probably the changes in the looks of the Klingon foreheads, or to be more precisely the messed up continuity through DIS. As much as I enjoy the show, this is one of the things I can hardly look over. I mean:
-TOS presents us the first variation of Klingons without their characteristic foreheads.
-TNG introduces the characteristic look of Klingons with those iconic foreheads. I don't know for sure, but I could imagine that back then people didn't ask too many questions on why they looked so different than in TOS. It was probably seen as a simple question of design and I think there was, unlike today, no urge to demand an in-universe explanation for everything.
-DS9 targeted the issue in the TOS-crossover episode "Trials and Tribble-ations". I only saw that episode once so far, but if memory serves, Worf is being asked why the Klingons looked so different back then and he says that this is something Klingons don't like talking about it. So the show implies here that there actually is an in-universe reason for the change but doesn't make a big deal out of it, seems like it's more meant to be a funny reverence.
-ENT, taking place about 100 years before TOS, brought the TNG-era Klingons back for no reason, other than maybe they thought Klingons looking like the TOS versions would irritate possible new watchers who only know the TNG-era Klingons. At least, in season 4 the show corrects its continuity error by creating a plot where the Klingons are given medicine that makes their foreheads disappear, stating that at some point they might return. So this fits perfectly into the universe and explains why they had their typical foreheads in ENT, didn't have them in TOS but have them again in TNG. Problem solved, as it seems.
-But then DIS came, taking place only about ten years before TOS (where Klingons don't have their typical foreheads) and had Klingons with their typical foreheads, probably for the same reasons they included them in ENT.
So, while it had been explained why the typical foreheads were there in ENT, were not there in TOS and were there again in TNG, it makes zero sense that in between ENT and TOS they came back in DIS and disappeared again about ten years later in TOS.