When you place water where there is already water (still or flowing) it places it, but it acts like it's breaking the block of water. It also looks like it when dropping the little pieces on the ground like breaking any other normal block.
MC-19626
Placing Water in water is messed up...
Resolved
Duplicate
0
Geo2236
(Unassigned)
Unconfirmed
water
Minecraft 1.6.1
Retrieved
56th Duplicate of MC-18365, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being closed as duplicate.