When working with DataTags I got the error message "Unbalanced brackets". I checked it, but all the brackets seemed to be okay. So I checked the error message again, but the message said "/summon SnowMan" even tough I actually had "/summon Item" in the command block that I got the message from. Turns out the one command block conflicted somehow with another command block I activated previously that had the "/summon SnowMan" on it, but worked fine ... If pictures are required, tell me!

PICTURES: http://imgur.com/a/QzWiu