As someone that has spent the last two years migrating legacy code, I can assure you, you do find bugs in legacy code.

I've seen bugs that user's don't notice.

I've seen bugs that user's have just gotten used to.

I've seen bugs where that failures it caused were dismissed as the system being a bit temperamental, for many years.

I've seen bugs that financially affected thousands of customers, and remained unnoticed over the course of several years.

I've even seen bugs that only remained because, by luck, the preconditions to trigger it had not been met.

I can assure you, you do find bugs in legacy code.