Granted I am speaking from experience on much lower tech machinery than 777s and GE
High-Bypass turbo fans, but, as a general rule, if a mechanical system is going to fail, it will normally happen very quickly.
What I mean is, when you build an engine, which is by it's nature, a complex object, you have to take into account that there are a lot of parts (which have to be fabricated, machined, built, etc) and then those parts have to be assembled into a final working unit. There a lot of places for something to go wrong, be it a defect in materials. A problem w/ the machine fabricating a part, or assembling the final product. An assembly line worker who was out late drinking the night before (Scary thought, isn't it)
Due to this, when you put all this stuff together, and power it up, anything that is wrong will generally manifest itself very quickly. In this example, a loss of oil pressure, it could be a gasket or oil seal that was manufactured incorrectly, or installed wrong, or a flange seal that the bolts weren't tightened right (correct torque is very important!)
How often does it happen? Not very, mass production is an amazing thing. But mistakes do happen. Face it, somewhere a human had to be involved in building that engine, and humans make mistakes. It's called "being human"
That's why you test run things. And why manufacturers provide "limited warranties." When I fix a machine at my shop, I don't just return it to the customer. I run it, and make sure everything is ok. And sometimes, guess what, I made a mistake. So I fix it, and then return the machine to it's owner.
It's not a big deal. I'm sure GE
has a warranty
on their engines. Fix it, test it, deliver it. Happens every day.