In survival mode, using a water/lava bucket has a slightly longer reach distance than block placement. You can reproduce this by digging a 3 block deep hole and then trying to place a block/bucket inside (pointing at the bottom block, no sneaking). This works despite the bottom block not showing a black outline.

The attached screenshot shows this situation: right-clicking will place water despite the lack of a black outline, and despite the player not being able to place a block.

The same problem also occurs with a lava bucket.