I was simply flying underground in game mode 3 (spectator mode) when I happened across 3 dungeons. I was in a 14w05 generated world, and I expected to check out the loot.

Not only were there no chests in any of the three dungeons, a commonly reported issue, the monster spawners (2 of them were spider spawners and one zombie) were not moving or spawning mobs. The mob icon in them was staying still. As soon as I switched to game mode 1, creative mode, the spawner icon started moving and mobs began spawning. When I went back to spectator mode, the spawner icons began glitching and monsters stopped spawning.

Steps to Reproduce:
1. Locate a dungeon in gamemode 3.
2. Observe. Switch to gamemode 1.
3. Switch back to gamemode 3.

I hope this is reproducible, and happy bug hunting/code wrangling to all you lovely volunteers/Mojang staff!