Back in '91, when Navy Arms was bringing the Brazilian 1937s in, I bought one. N frame Smith, what could be wrong with it? I shot it a few times, at the side of the road. Single action, at a 2 liter bottle. Worked great.
Two weeks later I took the concealed weapons course, and decided to use it. First shot fired, then two and three didn't. Hmmmm. Tried again. 1 out of 6 fired. Fortunately I had my Gold Cup with me, so I got the great joy of declipping 8 moon clips (by hand - I didn't know they made a tool back then) so I could finish the course.
Pondered over what could be wrong with that thing for almost a year. Worked fine single action, but would not fire, reliably, double action. Finally I had a brilliant idea. Took the sideplate off and sprayed the innards with Trike. Watched the melted cosmoline (some type of congealed grease, anyway) come running out. Works fine, double action, now. Woulda sucked if I'd needed to shoot a bad guy, though.
I don't care how well you think you know a gun, or whether it's new or used. Don't trust your life to it until you actually know it works.