Usually an oil change goes without a hitch. It is, or at least should be, part of your routine preventative maintenance schedule. If you refer to your car's owners' manual you will see when the manufacturer recommends how often you should have an oil change--typically every 3000 every 4 months.
Sometimes, when doing an oil change our auto mechanics will discover there are some repairs that are needed. This won't happen every oil change, but it is not at all uncommon. Sometimes the only way you know you need the work done is by actually performing an oil change. Kind of like when your doctor has your blood drawn so they can tell if you need something that they wouldn't otherwise be able to see.
Problems with the head gasket or O rings are the issues most commonly diagnosed during an oil change.
A blown head gasket can result in major problems in the engine. This will affect how your vehicle performs, you may notice that the car's performance isn't what it used to be, or that it is running rougher.
The O rings seal the oil filter. If an O ring goes bad then your car is probably already leaking some oil. A small leak might not seem like a big deal, but it is bad for the car, could result in engine damage because you run low, or even out of, oil between oil changes. On top of that, leaking oil is not good for your driveway or the environment. Some of that oil will leak into our water drainage system--definitely not good.