Off the top of my head the only times not knowing what's going on at the layer below has been a problem: 1. The layer below has a bug that manifests at the top

Off the top of my head the only times not knowing what's going on at the layer below has been a problem: 1. The layer below has a bug that manifests at the top layer. You're not sure if your code is broke or if the stuff your code is built on is broke. I see this a lot in the Java world where people use frameworks / libraries they're sometimes not even aware of they're using and it has a bug. 2. You run into performance problems because of the things you're using at the layer below you. C++ STL containers is a good example. It's pretty much black-magic, have you ever looked at the implementation? I also see this a lot in the Java enterprise world. One of the selling points of Java is that it's monkey-coder friendly. Except when it breaks and the monkey doesn't know how things work under the hood. Other than that, you should be fine not knowing the layer below you.