Ok, so I think this qualifies as a bug, though I'm not actually sure. Either way, I brought it up on #showdown the other day and got no response, so I thought I'd post it here.
Basically, I believe there is an issue with egg move illegalities. Now, for the most part, such illegalities are a thing of the past, but that is not entirely true. The change this generation was that females can pass down egg moves as well as males, and as such, any illegalities caused by needing two different species as the male parent are now eliminated. But there are two other cases where egg moves can be illegal together, and nothing has changed regarding those this generation. As such they should be still illegal. However, as of now, such combinations are legal on PS. Now, of these two, one is purely theoretical, as no illegalities involving it exist. But the other is actually real and relevant.
The first (and actually important) one of these is the case of baby Pokemon. For many of these, it is possible to hatch both the baby, and its evolved form from eggs, depending on held items. Additionally, these evolved forms may have different egg move lists from the babies. In certain cases, there may be moves on each list that are not on the other. These moves should be illegal with one another. To give an example, Azurill has the moves Fake Tears and Sing as egg moves, but Marill does not. At the same time, Marill has Amnesia and Aqua Jet as egg moves, but Marill does not. Thus, if you were to breed Aqua Jet onto a female Marill and then breed it with a male Clefairy that knows sing, you would have one of two results. If Marill is holding Sea Incense, then an Azurill would be born. It would know Sing, but not Aqua Jet, since that is not an egg move of Azurill. Alternatively, if it was not holding the incense, a Marill would be born, and it would know Aqua Jet, but not Sing, since that is not an egg move for Marill. In any case, the two moves will always be illegal with each other.
To put it more simply, the change in egg move passing has indeed made all egg moves of a given Pokemon legal together (baring the odd case below), but not all egg moves of a given family. Moves exclusive to one member of a family's egg list are illegal with moves exclusive to another members egg list. However, that is not currently the case on PS.
Now, while I know this is unimportant (at least at the present moment), as long as I am talking about it, I might as well mention the other, theoretical case: Volbeat. Volbeat is all male, but can be hatched via eggs from an Illumise. However, Volbeat and Illumise have different egg move lists. As such, if there existed two moves on Volbeat's egg list, but not on Illumise's that could not be passed down from a single male parent, then it would be impossible for them to be legal together, as the female Illumise could not hold onto the move for passing. Now, as it currently stands, the only egg moves Volbeat has that Illumise does not can all be passed down at once from Spinda. However, should there ever be a future case where Volbeat gains an egg move that Illumise does not have and Spinda cannot pass, illegalities would come into existence (technically, this is also the case with Nidoran-M, but since that is in the field egg group, it has Smeargle to eliminate all illegalities).
Anyways, this came up since we were talking in #cap about the potential for move illegalities now that egg move stuff changed, since they can have important competitive implications. While I'm not sure there is anything important being allowed now that should be illegal, since the idea came up, I just figured I'd drop it here.