There are three kinds of errors in a system, namely critical errors, non-critical errors, and unknown errors.
A critical error prevents the system from fully satisfying the usage scenarios. These errors have to be corrected before the system can be given to a customer or even before future development can progress.
A non-critical error is known but the presence of the error does not significantly affect the system's perceived quality. There may indeed be many known errors in the system. Usually these errors are listed in the release notes and have well established work arounds.
In fact, the system is likely to have many, yet-to-be-discovered errors. The effects of these errors are unknown. Some may turn out to be critical while some may be simply fixed by patches or fixed in the next release of the system.