I was doing some testing with spawners and spawn eggs and I noticed that when you are in spectator mode, some of the mobs that are in the spawner itself still have strange animations - animations unlike the ones in survival mode. As far as I'm aware, there should be no animation in spectator mode.
The ones that I found were blaze and silverfish. This does not happen with cavespider spawners. I am unsure how to make videos in game so try it for yourself!