I was using a world on 15w34b, and I think I exited it in gamemode 1, but I can't be sure. Upon opening it in 15w34d (I skipped c) I seemed to be in gamemode 3, but I was able to fly and stop flying (And I would sink through the ground slowly as though I were in water) so I always had noclip, and I was not able to change my gamemode via commands (Though I did not check my NBT data). So when I typed "/gamemode 0", nothing would happen. Same with all the gamemodes. I switched back to 15w34b and changed my gamemode to 1, loaded again in 15w34d and was fine.

I cannot reproduce this issue. I know this isn't enough information to solve the problem, I'm just posting this here in case it comes up in the future.