Without their knowledge?
A new bike showed up in the shop yesterday so I checked it out. It was a mountain bike and the front brake wasn't working. I tracked down the owner, he has commuted via bicycle before and I mentioned the brake. He kinda dismissed it. Well I just went to the shop to put my lunch container into my pannier and there was his bike right next to mine, still with a non-functional front brake. I decided to take a quick look. Seemed like the front cable stop was missing. No, it was there, it had just pulled back into the housing. I pulled it out... yada yada yada, one thing led to another and next thing I know, I found myself testing and fine tuning the brakes. I'm pretty sure he'll appreciate it but for some reason it still seems kinda wrong.
His back tire was also on backwards, but I think I'll let that go.