This is an strange and really stupid and funny bug, I didn´t try it in 1.8.x versions, I had tried it in 1.6.4 and in 1.7.10 but I am completly sure this is not reported before because it is very very strange.

If you get a zombie pigman spawner (I have used NEI but it is a minecraft item) and if you put it in a normal place, nothing special happens, it works normally like an other spawner. But if you put in ton top of a fence (I had tried it with normal fences and nether fences an it is the same), but if you put the spawner on top of a fence but clicking in an other block the bug doesn´t appear, you have to put the spawner on top of the fence clicking on top of the fence by clicking on top of the fence, and then it doesn´t appear a normal zombie pigman spawner, it appears a pig spawner!!!
I didn´t try bugs with other spawners but I think that maybe there are.

Sorry if my english isn´t perfect but I´m spanish and I´m only 14 years old.

I hope you enjoyed that report because it´s very funny, it really happens, don´t think it is a joke because it´s true, if it doesn´t work for you I will send you a video or something.

Please put a comment if you have seen it. Bye!