Sometimes, mistakes in challenges go overlooked, even after going through the sandbox. I've searched the meta, but there doesn't seem to be a meta consensus on this specific issue.
In both cases, users posted answers following the incorrect specifications in the challenge.
By the time the challenge poster edits the question to include the correct information (but not invalidate existing answers), it may be too late, and there would be two sets of answers that follow different specifications. Plus, having these two sets of answers looks bad since some answers would technically be "incorrect".
When one encounters a challenge that contain incorrect information, what is the best course of action?