Sometimes items won't pick up. So far I've seen this happen with material blocks and saplings dropped by lead decay--in other words, only items generated by block destruction. It happens somewhat rarely, but it's bothersome when it happens to a rare item. I had noticed this in snapshot 12w50, as well, but didn't get the chance to record it, and the field above wouldn't let me enter it.

Video evidence (smooth sandstone block):
http://youtu.be/eFvZt_-_S_Y

Update: After a Saving&Quit (but not shutting down the client) and then reloading the world, the dropped smooth sandstone item had disappeared from what it had fallen--and reappeared snagged on a block higher up! It's as though the collision-item somehow got separated from the graphical item: one stayed snagged on that block, and the other fell down but since it was only an image with no collision parameters, it couldn't be picked up. This may be related to the apparently-invisible harvested vegetables I had mentioned in the 1.4 bug thread on the forums.