I'm a big theory guy myself, and this seems to be the place for the theory of practicing, so please bear with me because the questions will come soon . The theory is that we can practice till our hands are numb and our mouths are dry practicing those "gets-em-every-time" one-liners, but then it's time to perform. These are routines you've performed tons of times before, and seemingly endlessly rehearsed in your "Magic Room". However, today something goes wrong that is out of your control.
Example: You perform a coke bottle vanish and glass of water production in and from a brown bag respectively (Fielding West Style. We'll call this effect "Brown Bag" for example purposes) You put the bottle in the bag for the last time, lift up on your glass but it comes unglued from it's stem. (for travel purposes I use Party City style plastic wine glasses, glued down to avoid this very incident) Again, you've performed Brown Bag tons of times to great result using the exact type of glass and coke bottle, but unfortunately it falls flat as you just pull out a stem with water and the coke bottle barely stays in the bag due to the glass coming undone.
How do we practice for these things? How do we say we have a plan B when the mishaps are usually so specific to the environment? On this day, the humidity was probably the cause of this incident. The same could be said for audience participation specific to the environment. Once again, you can practice your audience control lines till your mouth is dry, and they work in the shows 99% of the time, but there is always that 1% that is the variable. How do we practice the unknown?
I know one of the main things of show business is that it never goes exactly as planned during a performance, and you must be prepared for anything, but how do we rehearse something we can't force to happen in a practice environment? I mean, we can certainly be prepared for mis-haps, but:
A) How do we force these things to happen in a rehearsal environment when they are spontanious? Like this glass coming off the stem, I could never rehearse that because it just doesn't happen in a controlled environment, and without humidity the glass stays together. I hope I am being clear enough with this specific question, and I don't mean to get redundant.
B) Be prepared for something BIGGER than what you anticipate to go wrong?

Thank you for staying with me through this post, and I hope it sparks some good discussion!
Matt B.

I don't know if this is related to what you just said, but take for example the time I did a show, and practice and rehearsal was intense. NOTHING was going to go wrong.

So I thought.

Then to my utter chagrin, as I was performing, (on a carpeted floor) somehow, and I don't know how to this day, my shoe got snagged on the carpet. And the result was that it RIPPED the sole off the bottom of my show, about halfway, and for the rest of the show I performed, and walked, with my sole flapping in the breeze with each step I took. (My toes sticking out)

How do you plan for something like that?????

Or one time, I was going to perform Anderson' newspaper tear. It was my last effect, and I had the newspaper sitting on a side table.

Well, as Murphy would have it, it was raining hard that day, and the roof of the building I was performing in had a hole, a leak in it, And where was that leak (unknown to me) located?

You guessed it. Right directly above my table with the Newspaper. And then all of a sudden PLOP! A HUGE gush of water poured down from the roof, and like a bulls eye right onto my prepared newspaper.

Ever tried to perform the newspaper tear with a soggy, wet paper?

I have.

I can guarantee you won't dig it.

Oh, yeah, and also in that same show I caught my hand on fire, So I used a burned hand to perform a newspaper tear on a soggy newspaper.

Makes you think someone up there is out to get you.

So how do you prepare for these things????

Oh my goodness, I feel so bad for you.... I haven't been damaged by a prop yet, fortunately. Although I certainly hope not, it is certainly bound to happen to me as well! Anyway, that is a perfect (if tragically unfortunate) example of whatI am trying to present in this question. These issues are so event and site specific, is there a way to be prepared fully? Because laymen are so unforgiving and don't understand the challenges venues present, which then makes us look like we are not prepared properly. I am not pointing you out specifically, just in general, because we may do a show that gets a large ovation one night, and the next the unfortunate things both of us just mentioned happen. It seems to me that shows become obsticale courses you have to bob and weave through to get to the finish line, but how do we prepare for the next one?
Matt B.

I'm beginning to think you don't.

Murphys Law will always bite you in shows... We cant plan for every little thing that will happen on a stage or in the audience.
We can only be prepeared so much. Know when to cut a show short & also know when due to unforseen problems you may need to perform a bit longer...
the rest is up to the Gods.

Thinking on your feet, the law from Tamariz and improvising.

The law from Tamariz says if you get wrong the trick then you need to feel immensely good inside and tramsmit it to the audience then improvise with wathever went wrong that way its all part of the routine, you never fail. (I don't think this law is applicable to every kind of magic but it certainly is in card magic, andd this can be found in Magic from my heart by the maestro Juan Tamariz).

The repetition of perfection and accounting for all discrepancies will prepare you well for a performance.