When player (or mob) is in a block of water (source or flowing) and that block is touching the end of an 8-block flow of water and under certain conditions depending on the direction of the flow and world quadrant, it may be that individual can not exit the water from that block (except by flying in creative mode) or it may be that one can't exit toward the incoming stream but can in other directions. In still other cases, it is possible to exit as expected. The pictures and world download makes it clearer.
In the pictures the orange plus is centered on 0,0 (See Pic 1). In each of the four test areas, a central source block of water flows in four directions for eight blocks and stops at the edge of a 1x1 hole with a source block of water in it. The demo would also work if instead of a hole, there was a perpendicular flow of water (as shown in Pic 8). In some cases the hole is surrounded by red blocks meaning there is no way to jump out in survival mode (Pic 3). In two cases, no escape is possible even from an adjacent block if the hole is enlarged to 1x2 in the particular direction shown (Pic 5). A zombie has discovered the problem in Pic 6. In other positions escape is possible in any direction as expected (Pic 2). In some orientations, one can not jump into the end of the feeding stream, but can turn around and jump out (Pic 4).
A common Minecraft setup (Pic 7, 8) involves an area of flowing water that stops at the edge of a collection stream. Items dropped in the large area flow over the edge and continue to a collection point. Sometimes depending on the orientation of the flow and the signs of the x and z coordinates, one can not jump up one block from the collection stream to the edge of the flowing water.
This problem might even be useful as a trap (Pic 6) if it was not dependent on one's location in the world.
I've uploaded a demo world for testing: See Water Trap Demo.zip
This issue is clearly related to
MC-2324 and I was inspired to check the effect of the world quadrant and flow direction, and even to use colored wool to illustrate the problem, but my setup is very different: it works when exiting deep water or flowing water and and I'm not entering a source block of water blocked by a sign and in general it seems like a more practical or common setup. Please don't dismiss it as a duplicate; let both related issues remain open until the problem(s) is/are fixed and both set-ups are tested. The fix for one may not fix the other.