For minor and significant skill checks, you can roll against the appropriate stat if you don’t have the skill in question, but your stat has a –30 shift. If you make it, you succeed, but only barely – no finesse, no user-definable results, just a lucky success. Pat yourself on the back and find someone who knows what they’re doing next time.
For major checks, you can roll a Hail Mary against the stat. Only matched successes and crits succeed, and they don’t count as anything but plain old marginal successes regardless.
However, some things are just plain impossible. No matter how long you sit calmly in the pilot seat of a Firefly-class transport, without training, you’re just not going to get that boat off the ground.
Unskilled failure is usually worse than normal.