Here is the drop I used for an example:
<>
- action: MOBSPAWN
biome: DESERT
drop: SKELETON@eq:hands:diamondsword!!eq:head:sand/50%
<>

And being near a desert made one of these show up a dozen or so times a minute.
2013-02-28 14:48:13 [INFO] [OtherDrops:2.8b.54] ItemStack{SAND x 1}

Note: Lowering verbosity to low eliminates the message. Not getting any actual errors and the drop itself is working perfectly.

Found it - fixed in a local build.

Confirmed fixed in 2.8b.89