All non-trivial abstractions, to some degree, are leaky.

You can’t drive as fast when it’s raining, even though your car has windshield wipers and headlights and a roof and a heater, all of which protect you from caring about the fact that it’s raining (they abstract away the weather), but lo, you have to worry about hydroplaning (or aquaplaning in England) and sometimes the rain is so strong you can’t see very far ahead so you go slower in the rain, because the weather can never be completely abstracted away, because of the law of leaky abstractions.

The law of leaky abstractions means that whenever somebody comes up with a wizzy new code-generation tool that is supposed to make us all ever-so-efficient, you hear a lot of people saying “learn how to do it manually first, then use the wizzy tool to save time.” Code generation tools which pretend to abstract out something, like all abstractions, leak, and the only way to deal with the leaks competently is to learn about how the abstractions work and what they are abstracting. So the abstractions save us time working, but they don’t save us time learning.