If the task is especially difficult
At Grognardia, pondering the appealing/insane complexity of the rules for the old role-playing game Universe:
2. One of the character's Characteristics Ratings (specified in the skill description) is added to the square of his Skill Level, and the total is added to the base chance listed for the task. In some instances, the Skill Level may be increased (before squaring) by a piece of equipment or decreased if the task is especially difficult. The base chance may be further modified by the task description or at the GM's discretion (in some cases, he may apply a modifier secretly).