I have made a Mob spawner on my school's server, and what I noticed while building it is that the XP orbs wouldn't collect when I placed the hoppers in. The hoppers didn't need to feed into anything. The result was a never ending swarm of XP orbs around my head that only left when I moved away from the collection point of my spawner. What I have done to prevent this swarm is to simply remove the hoppers, leaving the loot in the spawner.

http://youtu.be/za7XT-3TbU0

In the video, it shows the difference between having hoppers below vs having no hoppers, and the problem being resolved by removing the hoppers