Anyone who has seen my solo live show knows that I have a lot going on on stage. I run multiple instruments and microphones through a mixer into my loop station and out to the house. I have quite a bit of gear setup on stage for a solo artist. I've played over 500 shows and just about everything that could possibly go wrong technically has. Even as I write this I know more shit will go wrong at an upcoming show that has never happened before.

Musicians are lazy. Many bands don't like rehearsing because they feel they have their set down and would rather play video games than practice. I'm not going to spend time here scolding you for that, I'm going to explain a practical reason why it's important to rehearse and run your set even when you think it's beyond perfect.

Technical issues are your fault - even when they're not. Your amp will crap out, your guitar cable will short, your batteries will die, your "built like a tank" Boss tuner will get dust in it and short out (happens all the time to me), the DI will die, the mic stand will fall apart and all of this you're going to need to know how to deal with on the spot, in front of your audience.
+The Sound Guy

It is never cool to say "we're having technical difficulties." Never say that. No one wants to hear it and it makes the audience uncomfortable and feel bad for you, in turn ruining your set. Even if the house PA system is exploding and you have 0 control over it you should know what's going on and address it accordingly (uh run like hell).

You need to know your gear in and out. You should know what it sounds like when your acoustic guitar's batteries begin to run out of juice. You should know what a blown tube in your amp sounds like. You need to know the difference between a shorted cable and a dusty DI. So when something happens on stage you know EXACTLY what it is and how to fix it. You should hope that the majority of your technical issues happen in rehearsal and you have a low stress environment to spend time and fix it and then know what to do quickly if it happens on stage. Soundcheck is extremely important as well to make sure all of the house gear being used for your set is working. If you skip your soundcheck or don't do a thorough check when you have the time to doso, then it's your fault when their gear craps out.

I once had a venue's DI crap out on my during my first song in front of a sold out show in San Francisco. I had just built up a 12 track loop with beat boxing, trumpets, bass, keys, guitar, the works! So when it crapped out it felt like Satan had just burst through the floor grabbed my sound and bust out the back door leaving only awkward silence.
++Watch Ari Loop This Song Live (YouTube link)

However, because I knew my setup so well, I quickly went through the check of everything it could be (checked the cable, checked the loop station, etc) and realized within 4 seconds it was the DI. Without missing a beat I got the crowd 1 2, 1 clapping while I told the sound guy I needed a new DI. He ran up switched out the DI and the sound was back and joined the 1 2, 1 claps almost right on!

I could have smiled awkwardly at the crowd, pissed myself then curled up in a ball on the center of the stage crying "there's no place like home" while clicking my heels, but that wouldn't have done anything. That's basically the same as saying "uh we're having technical difficulties" while looking awkwardly at your band members hoping someone will fix it for you.

Just last week at a show I tried rechargeable batteries in my guitar for the first time. I rehearsed with them and they were fine. Right before my last song (in front of 500 people SILENTLY waiting for me to start the song) my guitar started cutting in and out. I knew right away it must have been the batteries - but I went through the mental check list I've built up over the years when my guitar cuts in and out - tuner, cable, connection, etc and in 4 seconds I knew it must have been the battery connection with the rechargeables. I took them out, exchanged them with the normal batteries I had in my back pocket (just in case) and finished the show. All within 20 seconds. Felt like an eternity, but no one knew the difference while I told a story about having a dream about a chicken dance flash mob at a funeral.