Whenever breaking a block then in the middle switch to another item in your inventory it stops the block from breaking.
Linked issues
Comments 4
Duplicate of MC-2519
MC-2366
Breaking blocks
Resolved
Duplicate
0
Zack
(Unassigned)
Unconfirmed
block, multiplayer, world
Minecraft 1.4.3
Retrieved
When I tried this, at first the behaviour was as you described. But if I started breaking the block a second time, after it should have broken, it breaks instantly.