I would go so far as to say MOST mechanical failures IRL are due to people forgetting to check, or not doing a repair correctly... Hm. Now there's an idea... no idea if it's possible, but if a vehicle has been launched a few times, that particular vehicle gets an increased chance of failure, necessitating pre-flight checks. On that note, there's an idea. Pre-flight checks. Even if the increased chance of failure isn't possible, how about a pre-flight check. A kerbal must walk around the vehicle to check for problems; a damaged engine might have a little dribble of fuel or some visible damage.