[modulusshift]

A compiler in particular is a very extreme case. Most software is used to its fullest extent by its developer, 90% of bugs are likely to be found by said developer in their normal usage. Another step out, library developers often use their work, but often build in functionality they're not as interested in by request or as a result of generalizing beyond their specific use cases. They might not be the heaviest users of that software, and so aren't as likely to find some of the bugs.

Compilers are the most extreme case. Anything Turing complete is almost certain to have vast swathes of functionality which aren't used by the developer. If a compiler doesn't already have a large user base, then unless you're using it exactly like someone else has, you're almost guaranteed to find some bugs. Honestly that's true for most software, going back to the maxim "don't be the biggest user of any given software unless you're already capable of writing it yourself."