In real life, humans learn not only from good examples but also from bad examples if it is shown that the example is bad. However, the current way StackExchange works discourages any 'wrong answers'. Sometimes, those wrong answers are actually 'common mistakes' that others make and having it somewhere where people ask questions helps others learn more. (I've witnessed this on several occasions).
My suggestion is to have something like a CommonMistakes pin for this sort of answers, which allows others to see such answers, probably via a link or like in some cases on StackExchange, made a bit transparent.
Also since it's a common mistake, I'd say the actual reputation reduction for the user should be invalidated once it's voted so.
EDIT: I only posted this opinion, because I personally have deleted all my negative questions and answers even when they might have been helpful to another (Why leave my own sweat/hard work to hurt me even if I was wrong). So another option is to actually stop encouraging deleting of 'negative answers or questions' by restoring reputation when deleted.