I did a pretty thorough search and couldn't find any thing on this so if its a dupe I apologize.

If you hold down left mouse and try to mine netherack only one block is broken then you have to release the button and push it again to mine another. I first noticed it in a sync'd world, so I created a new world in 1.2.0.25 and it was affected also.This doesn't seem to affect any other blocks that I have found so far. (Using 2 different diamond pickaxes first one with fortune 3, E4 and U3 and a second pickaxe with Silktouch, E4 and U3)

after some more game play it appears that magma blocks are also affected and a diamond shovel with Fortune 3 has the same issue with sand dirt and gravel. silk touch is normal as well as any shovel without fortune on it