As far as I can tell, mods can't actually see when a deleted post is replaced with "This answer was marked as spam or offensive and therefore is not shown..." - we just see the original post. I'm glad we can see the post, but it could we also have an indicator that others can't see it? (even if it's just a kludgy version showing both the masking text and the post!)
When a post gets a couple not an answer/low quality flags and a mistaken spam flag, it's easy enough to just delete the post, rather than selectively marking the spam flag as incorrect and the rest as valid. But then other users will see the post masked, and we mods have no way to realize our mistake, short of examining the post history.