Now that you mention it, I seem to remember that when you catch a Ditto pretending to be your Pokemon, for some reason, the Pokedex didn't record its data. (In Gen 3 - my memory is a bit fuzzy but I believe that's the case)AND we're arguing over whether or not something happened to someone else. In Pokemon Gold. When said person says it happened.
Edit: Let's not forget that for a while, all Pokemon that used Transform were treated as Ditto, and wild pokemon that knew moves they weren't supposed to know were treated a bit differently. I have no clue whether or not this applies to Gen 2 or is restricted to Gen 1, since the only other Pokemon that knows Transform is Mew.
My facepalm- switching Gengar and later Mega Manectric into an obviously Scarfed Excadrill spamming Earthquake because I wasn't paying attention.