When placing a source or lava (from a bucket of lava) in a spot that is already occupied with lava (both source lava and flowing lava) the sound of breaking a stone block will play, and odd particles will come out. The type of particles are the ones that you get from a broken block, however they feature the lava texture... As if you broke a block of lava!

Note that I was using 1.6 prerelease at the time!