I was working on a Minecraft build just now, setting up a skeleton farm so I could have bone meal for my greenhouse. It was a fairly straightforward process... at first.
I had found a zombie spawner, and I knew that if you threw a mob spawn egg at the spawner, it'd change the spawner to that kind of mob. So, I switched over to creative mode, nabbed a skeleton spawn egg, and changed it to a skeleton spawner. That's when the mad scientist in me stirred.
I wanted to experiment and see just what I could do with a mob spawner, so I dug under a lake, hollowed out a cave under it, and set it up as a lab. I took a spawner and then experimented throwing various mob eggs into it to see just what would and would not spawn.
I finally ended up filling the entire cave with distressed ghasts who couldn't figure out where the heck they were and kept making these awful screeching crying sounds, so I simply dug out the ceiling, let the water from the lake start pouring in, and then sealed them in to their watery tomb on my way out.
I am a terrible person.