Occasionally, a situation comes up in my games that really has no bearing on skill, just on pure luck. Things that may be true, may not be, but are not affected by any skill or power of the PCs.
It's random stuff like the weather, the existence of some thing or place that has no particular reason to be or not be, or even circumstances where success is either automatic or impossible.
This is when we'd call for a "Reaction Roll from the Universe."
Pretty much, this is how it works.
Reaction Roll from the Universe. Make a roll on the Reaction Roll table. The more favorable the reaction, the more the Universe favors what is is you are hoping for. The less favorable, the less it favors it. Generally there are no modifiers to this roll, unless the character has some very broad bonuses to all rolls (extremely rare.) Do NOT use reaction bonuses or penalties; the Universe doesn't care about them.
You can use Luck normally here for re-rolls.
You can use this to set penalties, too:
Excellent: +10 to whatever you are doing.
Very Good: +5
Very Bad: -8
It's tough to get that +10 unless you have some bonus and roll an 18. Good luck with that.
Alternately, you can do this:
Luck Roll Against the Universe. This is a roll-low success roll. Roll vs. an 10. The more you make it by, the more the Universe favors the circumstances you're hoping for. The worse you fail by, the more the Universe undermines what you are hoping for.
You can use Luck normally here for re-rolls, and Serendipity basically lets you call a 3 here. I say basically because you use it instead, and it just works as written.
If you use this to set penalties, just count the margin of success as a bonus and the margin of failure as a penalty, for a range of +7 to -8. You can also just say a 3 is +10 and 18 is -10.
I've used the former for so long I can't remember when we started. It comes up rarely, but it's a handy mechanism we all agree on for resolving completely random and crazy situations. It's fun, though, and it's never a dull moment when someone asks something and the only real answer is, "Make a reaction roll for the Universe."