A spider spawner will start to spawn skeletons with no AI when the player is near it and if spiders are not able to spawn. The skeletons can spawn inside blocks or in the air regardless of light level or other obstructions. They have a hit box for the player and other entities but not each other. They cannot be moved. They can be hit one time, will take damage (particle effects and noise) and then can't be hit again. The phantom skeletons will spawn regardless of what difficulty level the game is on. Will spawn upwards of 200 entities. The only way I've found to get rid of them is to exit the game and reenter, though they will immediately start spawning again. Getting rid of torches so that spiders can spawn again also stops the skeletons from appearing. I've found two spider spawners, each acting the same.

I edited the custom world options on the world I'm playing in to have the max amount of spawners and went playing as normal.

Skeleton spawners are alright as far as I can tell. Zombie spawners continuously spawn chickens regardless of light level.The chickens act like they are being ridden by a child zombie and can actually do damage to you. Exiting and reentering the game makes 4-5 child zombies spawn regardless of light level. Then more chickens will spawn.