In 18w16a if you place water on top of a iron bar, it won't flow, however, in 1.12.2 it can flow.
Here's a video of it: https://youtu.be/YgRuXEKIVgg. Sorry about the lag.
Water placed above slab doesn't move
Snapshot 18w19a | Water source not flowing out in any directions
Water source remains stationary on ladders
Fence doesn't behave correctly.
Water does not flow, when placed over a sign
Water doesnt flow when placed above special blocks
Water will not flow over blocks with a waterlogged value of false.
Water flowing onto waterlogged blocks spreads outward, rather than stopping