Often I introduce far worse possible consequences than I would have comfortably decided upon by fiat, cushioned by the unlikelihood of rolling that nat 20 for the outlier result. I have had d100 rolls to end the universe, and d20 rolls to blow up the entire party to smithereens. For the random rolls that contain truly horrific outcomes I delight in making a party member do the actual rolling. So you want to take the time-travelling hummingbird-masked sorcerer up on his dare and jump into the lava tube down to the magma chamber like he did? You plan to replicate the complex ritual he did with his bad hummingbird self as he fell and jump into an existence outside of time and space like he did? If you roll a 1 you are going to be dead with no possibility of a raise and all your equipment will be lost. You still have a chance to chicken out... roll a d20...
During the last session of the 4e game I run I had several out-in-public rolls involving a rift to the future and a robotic incursion into a sacred grove in a volcanic crater.
16 or higher, more of the future robot mining machine comes through the time rift, 15 or lower, it stays put.
10 or higher, the time rift to the future grows wider, lower than 10 it shrinks.
1-3, the robot dragonfly shoots a missile at Beautiful Bob, 4-6 at Hammer. (I like d6 throws for determining who in the party gets attacked. Always have.)
I am sad to say that I used my nightly d30 roll for the first example last session and did not manage a 16 or higher (to my chagrin).
Likewise I failed to roll a 10 or high on a plain ole d20 attempt at the second example.
Next time, my pretties!