Sometimes when running a game you can just handwave the dice rolling part and do whatever makes most sense in a dramatic or storytelling context. Let the heroes win, against any odds. But sometimes, for the sake of uncertainty and dramatic tension, you want to let something ride on the dice.

The result might not be what the players want to happen, but often moments like this are looked back on fondly in later years. Failure can be as dramatic and memorable as success. The dice provide the means to fail, where a hero would normally succeed. The chance of failure makes success more sweet, but carries this price.

Transcript

Qui-Gon: I cast Suggestion to make him stop parrying my blows.
[SFX]: Krrzzrzrk!
Obi-Wan: For the last time, it's not magic. Don't say, "I cast such-and-such". Use the Force, you idiot!
GM: He rolls a 20 to hit! You need to Parry at -3.
Qui-Gon: Ooops.
Obi-Wan: What?
R2-D2: What?
Anakin: What?!
Qui-Gon: I rolled a 1.
{the mysterious black-robed figure stabs Qui-Gon}
{long drawn-out silence as Qui-Gon collapses to the ground, accompanied by a horrified reaction by Obi-Wan}
R2-D2: Should have pre-rolled the ones out.