Many mistakes while editing are currently detected automatically by either MediaWiki itself or by on-wiki template/module code. Currently, in almost all cases, this is done by adding the page to a tracking category (such as Category:Pages with script errors or Category:Location maps with possible errors). Many editors don't preview pages before saving and don't notice these categories, so they have no way of knowing that they made a mistake.
To resolve some of these cases, I suggest that we implement a __WARNCATEGORIZE__ magic word. When added to a category page, any time that saving a page would place it in this category, and it wasn't in this category before the edit, a warning would be shown to the user before they were able to save the page, in the same way as the warning for missing edit summaries currently are to editors with that option enabled.