To reproduce:
1. Rename Unity installation folder or one of its parent folders so that the full path would contain "test" substring. For example,
C:\Program Files\LaTEST Unity Version\...
2. Run the Unity
3. Create new project
4. Create new C# script file via Right mouse click -> Create -> C# script
5. Notice that it has been automatically placed inside Editor folder

Regression from 5.2 to 5.3

Code causing bug: http://forum.unity3d.com/threads/5-3-released.372567/page-7#post-2438471

Workaround: rename Unity installation path that it would not contain "test" keyword