The bug
Some non-full blocks that are washed away by water or pop off when placed into it (i.e. cannot maintain block form when submerged), show the flowing water animation at their sides briefly before they are destroyed.
Affected blocks:
Buttons
Carpets
Cobweb
End rod
Flower pots (with and without flowers/vegetation in them)
Grass
Heads/skulls
Lever
Redstone comparator
Redstone repeater
Sugar cane
Tripwire hook
Vines (simply destroyed on impact)
Related issues
is duplicated by
Comments

Personally I expect all these blocks to be WAI, so I've gathered these from the related reports into here so they can be judged separately.