Affects Version/s: Minecraft 18w07c, Minecraft 18w08a, Minecraft 18w08b, Minecraft 18w09a, Minecraft 18w10c
Fix Version/s: Minecraft 18w10d
Prior to 1.13 "aquatic" snapshots, a non-source water block would convert to a source water block, generally, under two conditions:
(1) The non-source block was horizontally adjacent to two source blocks, and
(2) The non-source block was above an obstructing block, which included both solid blocks and source blocks. (For the purpose here, an obstructing block is a block that water would flow "around" when determining spread.)
As of 18w07a, the second condition has been relaxed, such that the block beneath the non-source water block can be either an obstructing block or another non-source water block. (Note that a non-source water block still doesn't seem to act as an obstructing block.)
This makes certain kinds of cascading water effects (such as horseshoe water falls) impossible.
As mentioned in the linked reddit thread, this carries with it potential for large-scale griefing and attacks to client/server stability.