User blog comment:Kirkburn/Technical Update: March 25, 2015/@comment-24473195-20150327161427/@comment-452-20150328162012

As I said, not all "detected" errors are actually errors.

In addition, I often find that the error marker doesn't go away after I fix an error, and sometimes repeats all the way down the page with the same error, even after it is fixed.

So, preventing me from saving the page after I've fixed the error because the error-detection thinks there is still an error would not be very helpful.

But hey, Wikia already prevents MediaWiki:Wiki-navigation from being saved when it falsely detects a width issue, so maybe they'll implement your suggestion despite the occurrence of false positives.