Three examples:
DRY (Don't Repeat Yourself) sometimes leads to premature abstraction. We think, "hey, I bet this pattern will get used elsewhere, so we need to abstract out the common parts of the pattern and then..." And that's when the Complexity Demon enters.
We want as many bugs as possible caught at compile-time. But that means the compiler needs to know more and more about what we're actually trying to do, so we come up with increasingly complex types which tax your ability to understand.
To avoid boilerplate we create complex macros or entire DSLs to reduce typing. Unfortunately, the Law of Leaky Abstractions means that when we actually need to know the underlying implementation, our head explodes.
Our challenge is that each of these examples is sometimes a good idea. But not always. Being able to decide when to introduce complexity to simplify things is, IMHO, the mark of a good software engineer.