Sometimes, things go wrong with bikes, which you just can't understand. A headset that you just adjusted comes loose withing a block of riding, or or a newly replaced tube is flat before you even get the pump stowed away.
Usually, you can find the cause, if you look closely, though.
One of the guys in the shop came out, and quietly asked me to look at a bike in the stand. I excused myself from the customer who had just bought the bike, and walked back.
"I don't know what's going on," the mechanic said. "The bike was shifting fine, and now the rear shifter won't even move."
I looked at the bike, spun the cranks and tried the shifter. Sure enough, it seemed that the shifter was frozen. Five seconds later, I thought I knew what the problem was.
"Did you install a kickstand?" I asked.
"Take it off and put it back on. You clamped the cable to the frame, with the kickstand," I told him, as I walked out.
"You didn't even look at it! How do you know that's the problem?" he called after me.
I had tried the shifter, and it was locked. Then, I had pulled the cable, itself, to see if the derailleur would move. I could see that he cable didn't move, between the bottom bracket and the derailleur, when I pulled it at the down tube.
I saw the mechanic look under the bike, and the expression on his face, when he saw the cable clamped down, let me know that he would make that mistake, again.