We're all familiar with the principle of Murphy's Law/ Sod's Law – if it can go wrong, it will go wrong. And us musicians are in no way exempt from it's lugubrious dead hand. The broken string at the climactic bend of our spotlight solo, the amp catching fire just as you're kicking into the big finale, the compere's oversized clown shoe tripping and unplugging your amp so that you launch into your big intro with NOTHING... (yes, that last one really happened to me).
An amateur guitarist worries “What happens if X or Y goes wrong?” and ties himself in knots trying to plan ahead for every contingency.
A professional guitarist knows the whole thing is going to go to hell, almost immediately, but grits his teeth and gets on with things anyway.
There are a whole raft of tiny, low-level teething problems that can disrupt a performance – things that simply cannot be replicated in a rehearsal room or a lesson. I think the problem really lies in our expectations – when we see a band, live or on TV, all the background stuff (cables, DI boxes, mixing desks) is hidden away, out of sight. The result being that people think music just happens. But when you stop to think about it, there's some very complex logistical and technical processes going on. It's only natural for their to be some issues along the way.
You pick the string. The vibrations are sensed by the pickup (assuming you've selected the right one and not accidentaly turned your volume down for any reason) and turned into an electrica impulse.
This electrical impulse is transmitted through the lead – assuming your jack socket hasn't worked itself loose, assuming there are no bad connections in either the jack socket or the tip of the lead itself – through to the amplifier.
The amplifier then adjusts the signal, adding gain, editing frequencies and running through a reverb chamber – assuming there is no issue in the input of the amplifier or in any of the circuitry, assuming the valves (if applicable) are in working order and have not burst or degraded - and then sends the amplified signal to the speaker which vibrates to create the soundwaves (assuming there is no damage to the speaker (for instance if it's been accidentally smacked into a chair leg... or table corner.. ask me how I know)
For some players the chain ends there. For others there is a mic'd or DI'd amp – which necessitates another cable, a DI box or mic.. another link in the chain, another potential source of problems. There's also the possibility – probablility – that there's at least a couple of pedals and/or a multi-FX unit in the mix. That's patch cables, batteries, power supplies – patch cables can break, batteries lose charge, power supply units are notoriously bady designed and badly made, and vulnerable to issues in the venue power supply. I vividly remember a gig back in 2009 with my new Line 6 FloorPod XT (I think) where the damn thing would NOT stop buzzing – could I replicate the issue at home? Could I hell. As it turns out, the problem was the power supply at the pub we had been playing at. An earth loop issue in the pub's wiring was the problem. Without a ground lift function, there's nothing you can do but grin and bear it, and crank your noise suppressor...
Then there's the cable. You WILL wind it round your ankle. You WILL catch it on a mic stand. Unless you gaffa tape it down, it WILL get stuck under your wah pedal. Wireless? Batteries. Interference.
There are always going to be problems when you play live. There is no textbook of solutions. You just have to be on your mettle, able to respond an d improvise your way out of the situation. And for that, you need experience. Sometimes, that experience will by necessity be embarrassing – and I'll see whatever your story is, and raise you the one where I set my own hair on fire trying to do a Slash and smoke on stage – but there's no getting around it, it has to be done. You experience the problem, you find a solution. You can't short cut it, you can't skimp on the learning curve – but be aware, every story will eventually be something you can laugh about over a beer a few years down the line.
Take your lumps, learn your lessons, and enjoy the ride!