As the topic states, with only Forge installed there are snowballs created when water is flowing over snow layers. In vanilla Minecraft the water just destroys the snow.
The bug occurs in 1.5.2 with the newest Forge version #696 (and i remember this happening long time ago with various mods installed).
I use MCForge to install some client-side mods like a minimap and looking for an automated snow farm. I discovered that water breaking snow creates snowballs in creative singleplayer, so i made a machine. When i had it rebuilt on a vanilla server it didn't work. That's when i tried it with a new vanilla instance and then with Forge installed. Turns out that Forge modifies the game so that snowballs are created.
As Forge is just a modding framework that shouldn't modify any vanilla game design mechanics i guess this is an unintentional bug that should be fixed.