It depends on the context, but whether or not we allow you to repeat has less to do with realism/plausibility and more to do with not penalizing the player for encountering a challenge at the "wrong" time in his/her build. If most of the safes you encountered in the wasteland wouldn't allow you to pick them after the first time they were examined, that would be kind of irritating. The "you're going to fail" options aren't present in dialogue so the player can select them and see the wacky response to their incompetence (although that does have some appeal), but to let the player know that there's a challenge there that they can't currently meet. Unless you're being directly confronted (e.g. the ambush scenario you described), allowing the player to return and attempt the challenge with a higher skill is less punitive and does reward players who choose to go and advance their skills -- whether that's through leveling up, using chems, or reading skill magazines. Magazines and chems only provide a small boost though, so if the threshold is 80 and you have 33 in the skill, you're not going to be able to bridge that gap without some serious time and effort (i.e., leveling up).