Windows 10, gtx 970
tested in 2.78c and 2.74
Short description of error
Decimate causes errors with vertex colors. Games often put data in vertex colors to be used by shaders, in this example it is used to animate the stones appearing as they get placed by the units in a game.
Decimate isn't keeping this information as intact as it could be. In this example each stone has a consistent vertex color of 1 more than its nearest lowest stone on the up axis. (I think that the data is stored in the game asset is between 0 and 255, so the value is actually 1/255 more)
To demonstrate in the blend file I converted colors to weights and then use the displace modifier.
The error you can see in blender is mirrored in game
Exact steps for others to reproduce the error
Open blend file
Apply decimate modifier to object "pre decimate"
Re convert vertex weights using the script "convert" in the blend file
Toggle the displace modifier off and on to update it
Expected behavior is for object "pre decimate" to be the same as "decimate"