Roger Cicala at Lensrentals.com, err, nails it (sorry):

Hammerguru: Justgotmine – looking at your images those are pretty long nails. Were you using good technique with a nail stabilizer? It also seems the hammer wasn’t lined up square to the target. It’s impossible for us to help you if you don’t eliminate all the other variables. It could be the hammer, could be the nails, could be technique.

(This has been everywhere, but I first saw it from Tim Bray, retweeted by Jeff Atwood)