First of all, here is my log, if you need it: http://tinyurl.com/kmd9pe3
So, I was using a seed that my friend gave me, and went to a location where there was supposed to be a skeleton spawner, but it was a spider spawner. I then made a new world with the seed in a vanilla 1.7.10 world, and saw that it was, in fact, a skeleton spawner without Forge.
I then put just Forge on that vanilla 1.7.10, and made a new world with that seed again. It was a spider spawner. So, I figured it must be Forge causing this.
(To clarify, Forge is not changing a spawner that's already been loaded, its that it generates the spawner different from what it's supposed to be).
Here is a picture of the spawner without Forge:
and with Forge:
Here is the seed if you want to see it for yourself: 2138374464889032671