When I stand on a solid block that has a water source block on top of it (=feet level) and a sign on top of the water source block (=eyes' level), I cannot always climb up on top of a block that has (or not) another water source block on top of it. It is difficult to describe it in one sentence so I uploaded a video on Youtube: http://youtu.be/NI98uWkYm1o and I uploaded here the two test worlds. Better description below.

I notice that it occurs differently in each of the four quadrants of the world but it is identical at different spots in the same quadrant.

Case 1, the player tries to escape from water to air/air (feet / eyes level):
If the player is in the first quadrant of the map (x>0 and z>0) direction doesn't matter, he can exit to all 4 cardinal directions.
If the player is in the second quadrant of the map (x>0 and z<0) it is impossible to exit to the south side.
If the player is in the third quadrant of the map (x<0 and z<0) it is impossible to exit to the south and east side.
If the player is in the forth quadrant of the map (x<0 and z>0) it is impossible to exit to the east side.

Case 2, the player tries to exit from water / air to water source block / air (feet / eyes level):
If the player is in the first quadrant of the map (x>0 and z>0) it is impossible to exit to the south and east side.
If the player is in the second quadrant of the map (x>0 and z<0) it is impossible to exit to the south north and east side.
If the player is in the third quadrant of the map (x<0 and z<0) it is impossible to exit to all four sides.
If the player is in the forth quadrant of the map (x<0 and z>0) it is impossible to exit to the south west and east side.

(There is a symmetry of behavior between the second and the forth quadrant.)

So this bug renders mob spawner farms impossible to work most of the time.

EDIT:
About the attached photos:
Lime wool means that the player can escape to that block.