"Inept fumbling" is a good phrase to keep handy for your descriptions whenever someone rolls a critical failure. In fact, you should keep a menu of similar phrases to pepper your descriptions with. Use this handy table!

Roll 2d12:

First roll	Second roll
Artless Awkward Bumbling Butterfingered Clumsy Crude Ill-conceived Inept Lump-headed Maladroit Oafish Ungainly	Blunder Botch job Cock-up Faux pas Floundering Fumbling Goof Incompetence Miscalculation Mismanagement Screw up Slopwork

Transcript

Han: Is it my turn to roll some dice yet?
GM: Sure, roll for Hotwiring.
Han: Hooray, finally!
[SFX]: roll
Han: 1! Oh.
GM: Your inept fumbling causes the auxiliary blast door to close as well.
[SFX]: sshhwooom...
Leia: Okay, I see why you never use Hotwiring yourself.
R2-D2: I like that they didn't close the blast door to keep enemy forces out in the first place.
C-3PO: They were saving it for inept hotwiring attempts.
Trooper: Surrender! I'm firing a warning shot.
[SFX]: Pow!
GM: <roll> Oh. Princess, you're hit in the <roll>... arm.
Trooper: Sorry! I missed!