Keys can be surprisingly fragile. If you're not careful with your car key, you might accidentally break it and leave a broken piece of your key inside your car. If you aren't able to open and close your door using a key fob, you might find yourself trapped outside your car until a locksmith can arrive to repair your lock. However, there are fortunately emergency locksmiths who can get you inside your vehicle immediately.
Oil leaks on any engine can be a major problem. Like your car's engine, your lawnmower's engine needs a sufficient oil supply for lubrication and cooling. Running a small engine with too little oil will rapidly produce internal damage that will likely be far more challenging and expensive to repair than a simple oil leak.
Fortunately, it's usually relatively easy to spot oil leaks on a lawnmower. It's a good idea to give your mower a good cleaning several times per season, during which time you should check for signs of oil leakage above and below the deck.