He should have jiggled the gearshift to make sure it was in neutral.
When driving a vehicle that you're unfamiliar with, always pay extra attention. As an example, watch this video of a young man crashing a Ford Mustang Shelby GT350R into a garage wall.
The clip simply shows the car starting up, lurching into the wall, the young driver getting out, and an expletive on the screen. After some discussion amongst the Motor1.com team, we are pretty sure about what goes wrong here.
The GT350R starts up with no problem. We know that since the Mustang has a clutch interlock that the driver must have the pedal depressed to get the engine going. Then the car jumps forward. The most obvious cause is the young man thinking the car is in neutral, but it's actually in first gear. So when he takes his foot off the clutch, the vehicle would surge forward.
If nothing was in front of the Mustang or if its nose was facing outward, then this lapse in attention wouldn't have been a problem. Unfortunately, GT350R was facing the wall. From the cry the young man let out as soon as he got out of the car, he knew immediately that he screwed up. Avoiding this crash would have been as easy as jiggling the gearshift to make sure it was in neutral rather than in first gear.
If there's any good news here, it's that the damage to the Mustang is likely mostly cosmetic. The corrugated metal wall bends when the car hits it and probably absorbs most of the impact. The kid's dad is still likely furious, but in terms of wrecking your father's sports car, then things could be a whole lot worse.