I do a mag check after every shut down.

7 years ago i swhiched seats (R to L) in a 1985 C182 R model. The guy flying before me did a mag check before sthutting down the O-470. It was smooth and offered no indication of a pending failure. I started the engine about 10 minutes after the shutdown and successful mag check by the precious pilot. I taxied to the hold line and went through the pre-takeoff checklist. The left mag was dead. We tried to ressurect it, but no luck.