When trying to place a block in lava with flowing lava in front of it, if there isn't a solid block where you're targetting then the block placement acts as if the lava block immediately behind the lava flow is a solid block and places the new block in front of said lava block.  

Edit:

This is happening with water as well but a bit differently.. if you stand at the edge of a block and face into the water with no target then place a block, the block will be placed on the front face of the block on which you're standing.