Of course when you set traps for the PCs to encounter, you have to give them some way of detecting, disarming, or otherwise avoiding the trap. Or at least the appearance of a way to avoid it.

Conveniently, many game systems specify that - because a character shouldn't know if there is a trap there or not before a successful detection attempt is completed - the GM should make the roll to determine if the attempt has been successful, without revealing the actual numerical outcome of the roll to the player. Ostensibly this is because if the player rolls what is obviously a good result on Detect Traps and the GM says, "You don't find anything," that's very different from the player rolling a poor result and being told, "You don't find anything."

The real reason, however, is so that the GM can fudge the dice and declare that the PCs are trapped, even if he actually rolled really well.

Transcript

Obi-Wan: We head for the hangar.
GM: Okay, what are your Detect Traps skills?
Obi-Wan: Oh, not good.
Anakin: I'm more of a setting traps kind of guy.
GM: All right. You're caught by a force field.
Anakin: Wait a minute. How did this happen? We're smarter than this.
Obi-Wan: Apparently not.
Obi-Wan: I say patience.
Anakin: Patience?
Obi-Wan: Yes. This is R2's specialty. It's under control.
{R2 comes skidding across the hallway and bashes into the opposite wall}
[SFX]: Zoom...
R2-D2: Oh god oh god oh god, I can't see them, they must be right behind me...
[SFX]: Zoom...
[SFX]: Crash!
[SFX]: Bounce...
[SFX]: Spin...
Obi-Wan: See? No problem.