Is there a term to describe something difficult to find but easy to understand?

Your solution is "a needle in a haystack."

I'm pretty sure there is no single word to represent your phrase. English doesn't do single word complexities like that nearly as universally as people seem to think; that's what we have phrases for!

The best phrase I can think of is that your mistake was "obvious in hindsight," or variations on that theme. The idea that is expressing is that once you saw the line of code with the incorrect function call (however long or short a time it took you to get there), you immediately knew it was the cause of your trouble. This doesn't fit your second example quite so well, however, and it doesn't make a terribly strong statement about the problem being difficult to find.

I think you're attempting to express too many ideas for one single word to describe. The closest thing to fit that description would be "enigmatic", but that only vaguely implies a level of unforeseen complexity and not that the solution itself was simple.

With two words, you could say "deceptively simple" which fits the bill much better than one word could.