Title is mostly self-explanatory. For exactly one entire game, every single time I used E, it played the sound for Swain's Nevermove (W) ability.
CONFIRMED! This bug occurs in every game on Summoner's Rift, Crystal Scar, and Twisted Treeline, (can't test Howling Abyss alone, but I suspect it works there too) that contains both Veigar and Swain, regardless of team. Tested with both classic Veigar and Final Boss, with and against classic Swain. I have no reason to suspect that any skins make a difference.