It kinda a strange glitch, I found it totally by accident. But I am able to trigger it every time.

When you are running from Dr. Fetus at the end of the Forrest section.

It you hit the Windows Key while in full screen mode, then come back to game. (Control+Alt+Delete works too.)

Dr. Fetus will disappear, allowing you to basically run the whole level (if you can) with out Dr. Fetus chasing you. But as soon as you die and restart the level from the start, Dr. Fetus returns.