In my last blog, one of the scenario examples I offered was of players saved from a particularly daunting battle by the discovery of a trapdoor. At the time, I thought it was a bit of a stretch for the players to happen to discover the trapdoor just when they need it. But, on reflection, such an occurrence needn't be a coincidence (from the dev's perspective).
Adventure movies are full of such jaw-dropping coincidences. Indiana Jones and his father are tied to chairs as fire spreads through the room and threatens to kill them both...but when they hop to the temporary safety of the fireplace, one of them happens to hit a lever which spins the fireplace (a secret door) to a room safe from the fire. Hans Solo finds himself at the mercy of an Imperial Cruiser and its fighter battalion, but there happens to be an asteroid field nearby in which to dodge and hide.
Going back to the tomb-trapdoor scenario, imagine that the trapdoor would not be revealed until such a great level of danger is present. One variable is dependent upon another. The group might experience a particular dungeon several times before variables combine to create a situation of high enough difficulty that the secret escape route is revealed/opened to the adventurers. Note that this is not a form of instancing which divides the gameworlds of separate players or groups (the creation of a temporary universe, like some of the quests in EQ2), but instead just a toggling of accessibility.
I'm sure this could be applied to scenarios other than setting variation, but that's the most obvious application.
Perhaps another could be a weapon that has a chance of producing a particular effect (ex: cast [X] debuff, add [y]-type damage, etc) and that effect has a chance of interacting with other variables to produce an additional effect. If a weapon happens to apply its frost effect during a battle with a Coldstone Golem, the golem will shatter like something transformed with liquid nitrogen. If the weapon's application of its frost effect was uncommon enough, then players would not be able to count on it to chop through an area of Coldstone Golems with ease, but just the chance of witnessing the consequence of the combined variables again would be enough to excite the players with anticipation. If similar combinations were common enough, then all players, including those who had never witnessed such an effect, would enjoy similar anticipation.
Fun can be produced, not only by the things players experience, but by the things they can hope to experience. Anticipation and surprise are vital elements.