System Information
Operating system and graphics card
win 7 64bit, AMD Radeon HD 6500M/5600/5700 Series

Blender Version
Broken: Blender 2.69 r 4a14102, but probably even before this revision
Worked: Blender 2.69 release

Short description of error
When opened an older file I got this error as seen in the attached images. There's an N-gon which gets somehow different triangulation and thus it creates a face which goes diagonally across.

Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps
Just open the attached file in 2.69 official and a new revision.