- BAD:"We need to burn down the bridge? Well, I never told you this, but I've known how to use fire magic since I was three." (Magician gains the ability to conjure fire, bridge burned down).
- GOOD: "At this rate, the enemy's going to break through. I guess this calls for something a little more drastic." (Warrior's combat prowess increases, giving him a better chance of winning the battle).
Now having written that example, I guess what I'm looking for is something that will increase the odds of overcoming a given obstacle, but not be a sure-fire win and not come out of nowhere. Your response will be more useful is you can explain things rather than just say "Look a [mechanic X] in [system Y]."
A little bit of context--I'm working on designing a CRPG with some friends of mine, and the above idea would be very cool for certain parts of the storyline.