Operating system and graphics card
Win 7 64bit, ATI Radeon HD 5700 series
Broken: official 2.70, but was present in older builds too
Short description of error
Certain values of blur produce artifacts.
Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps
Just render the attached file and see the borders of the blurred area where it gets sharply cropped. This only happens with some values of blur. Also visible on attached image.