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.

Demo: https://www.reddit.com/r/Minecraft/comments/7z3zzx/for_those_who_havent_seen_it_the_new_water/

[Edit]

As mentioned in the linked reddit thread, this carries with it potential for large-scale griefing and attacks to client/server stability.