For the past several releases, I've noticed that mob spawners (such as in Dungeons or Mineshafts) will activate and spawn hostile mobs even when the server properties file is set to Peaceful. The mobs that spawn do not de-spawn and must be killed normally.
MC-9665
Spawers activating in SMP while on Peaceful
Resolved
Works As Intended
0
Unable to reproduce. Mobs spawned from spawners despawn immediately if difficulty is set to 0 (peaceful).