The Achievements Get Reset Whenever The Player Switches Jar-File(E.g. When Updating Minecraft). This Is The Only Condition I Know But There Are Many More, If Anybody Knows Any More Please Feel Free To Add.
Environment
OS Name: Windows 8 x64
OS Version: 6.2
Java Version: 1.7.0_09
RAM Installed: 8GB
Core: Intel Core i7
Opengl_Vendor: NVIDIA Corporation
Opengl_Version: 4.2.0
Duplicate - MC-583