When spawning a Music Disc (i'm not sure if this is also true for other items) via the /summon command using a command block, it can't be picked up by a player.

This is the command i used:
/summon Item x y z {Item:{id:"record_stal"}}

The y-coordinate was about 5-6 blocks above the ground. Strangely, when leaving out the NBT-data the stone block which is spawned in this case can be picked up.