Minecraft will create a water source block (spring) at an empty location if that empty location is adjacent (horizontally) to two or more water springs and the material below is "solid".
Minecraft currently has a bug that prevents a water spring from being created if the empty location is directly above another water spring. In the picture below, the empty location between the water springs will not become a water spring itself.
This leads to annoying currents and painfully tedious ways of fixing them. This mod corrects this problem, so that, in the picture above, the empty location between the water springs will itself become a water spring.
Download Water Propagation Fix for Minecraft 1.3.2 (Client and Server)
Details for developers and those who care