In Episode III: Revenge of the Sith, when Obi-Wan challenges Grievous to a lightsaber duel, Grievous replies:
You fool! I've been trained in your Jedi arts by Count Dooku!
This makes it sounds like he and Obi-Wan have never fought before, and he's gloating that Obi-Wan doesn't know what he's up against. But The Clone Wars firmly establishes that Obi-Wan and Grievous had already engaged in at least one lightsaber duel prior to this point. Obi-Wan already knows that Grievous is trained in the Jedi arts, and Grievous knows that Obi-Wan knows.
So why did Grievous say this? Why would he brag about something Obi-Wan already knew perfectly well?
I know the out-of-universe answer is a) the line was for the audience's benefit, and b) they hadn't written The Clone Wars yet, but I'm looking for an in-universe answer, if one exists.