What I expected to happen was:

using /blockdata or /setblock I tried to create a spawner which spawned "Item"

What actually happened was:

Minecraft crashed with a complaint about rendering entities in the world.

Steps to reproduce:

1: log into 14w32d or later

2: start a new world

3: using /blockdata or /setblock, attempt to set the EntityId tag of a mob spawner to "Item"

4: enjoy as minecraft repeatedly crashes upon loading the world.

Notes:
I originally attempted to create a mob spawner for an adventure map on a world I'd done a lot of work on. When I created this mob spawner, minecraft crashed and continued to do so upon loading the world. I was lucky and was able to destroy the mob spawner after logging in, just before crashing.

Command used:
/blockdata ~ ~-1 ~