Recently, I helped a fellow stacker investigating a problem he thought was with generated QR codes. The real problem appeared to be with HTTP server redirect codes:
The user deleted the question after leaving a comment to the effect that replacing the QR code generator 'solved' the problem. Deleting the question took away a decent answer that sadly had languished with no upvotes. (It's not great, but it did answer the specific question. I'd give it an upvote had I seen it in time...)
I want to leave a comment on the question explaining that deleting questions is impolite. But I cannot leave a comment on the deleted post. Since I'm just a user, I have no access to the moderator-only messaging system.
Please let us leave comments on deleted questions and answers. (I've wanted to tell people to undelete specific answers before; leaving a comment on another answer seems silly, but worked before...)