Slender: The Arrival

In order to fail at pausing, you need to be either looking at, or in close proximity to Slenderman. Normally while playing the game you can pause to change options, etc., but it's been made so that you will fail to pause (the screen will flicker and you can't pause at all) when in danger. This is meant to heighten the terror aspect of the game (people often will pause horror games as an escape from the danger).

You can easily farm the 50 failed attempts on any level with Slenderman. Simply look in his direction and mash the / button; if your screen is flickering intensely, you're doing it right.