Just like the title says. When holding items like ...

• bow
• sword
• flint & steel
• shovel
• pickaxe etc
  ... pick block (middle mouse button by default) does not work.

EDIT: It only does not work if your item bar at the bottom of the screen is filled with blocks that are not the block you try to pick.

f.ex: Your bar is filled with all colors of wool and one sword.
If you hold the sword and try to pick a stone the sword won't get replaced but if you hold wool or some other block it does.