Critical Input Error

Is there such thing as degree of incorrectness when it comes to user-supplied data? As in, you forgot your email address...we kinda need that. Is that a critical error? Is it an every day occurrence that we can expect as user interface developers? Perhaps, we need to design into the experience, and into the data ultimately consumed by the software, the fact that we'll get it wrong. Often. But how do you deal with the paradox of missing data? You can only provide so many friendly messages and so much hand-holding before your software languishes due to data starvation.