The most recent case of an illegal egg move was Simple Beam on Psyduck and Spoink, between SV launch and May 29, 2023. While both of those Pokemon had Simple Beam in their egg move list, no species that was available in the whole game had the move in their level up lists (until DLC2 when they brought Latios in), so there could be no initial donor to use the Mirror Herb with, nor did SV feature a mechanic where the Pokemon could be caught already possessing one of their egg moves. Only when Home compatibility was added, could you transfer in a Psyduck or Golduck that had Simple Beam from breeding in Sw/Sh, and use the Change Moves feature that was added to Home at the same time to import the moves from one game to the other.
Other examples of impossible egg moves that postdate XY's "egg moves are now inheritable from both parents" change:
Foul Play Spiritomb in BDSP - This predates the Mirror Herb, so the only form of egg move transfer that existed required both parents to be the same species. None of the Pokemon that were in the right egg group within the limited dex available in BDSP could get Foul Play, not even through chain breeding, and while Grand Underground Pokemon could spawn with a random egg move, Spiritomb is not one of the Pokemon that could be found there. As with Simple Beam, this combination became legal once Home supported the Change Moves feature when bringing a Pokemon to a new game, so that you could finally retain some Sw/Sh moves when importing to BDSP.
Punishment Murkrow in USUM - For some reason they added this to its egg move list in the Ultra pair; it wasn't there in the original pair. The only egg group partner that could get Punishment is Mandibuzz, which is female-only--meaning all potential pairings will only give Vullaby eggs, not Murkrow eggs that can inherit the egg move. Punishment was removed from all Switch games, so this combination has still never been made available.
