Sonicboom

[DaBlackBandit]

I was wondering why SonicBoom hits Gastly, Haunter, and Gengar. Since Sonicboom is a normal attack. But Swift doesn't?

[Poryhedron]

Well, in RBY it was because Sonicboom deals a set amount of damage (20) and thus ignores type relations completely. It just didn't occur to the game designers that set-damage moves that don't respect weaknesses or resistances should respect immunities. For the same reason, Seismic Toss works on Ghosts and Night Shade works on Normals.
I seem to recall Sonicboom working on Ghost-types even in GSC, though, when Seismic Toss doesn't. Am I right? If so, it's probably because it's not a contact move. Idunno.

[Lady Vulpix]

I think it doesn't work in GSC. I could be wrong, though. I know Sing works, but that's because it doesn't do any direct damage.

[mr_pikachu]

Some other moves also ignore such immunities, even today - and I assume they did in RBY as well. For example, Confuse Ray works on Normal Pokemon, and Mud-Slap (not in RBY, but it still proves my point) hits Flyers. Aside from Mud-Slap, most of the immunity-avoiding moves only inflict statuses or set damage, which may have something to do with it.