What are the most notoriously deadly scenarios and campaigns?
While I've never played it, "Tomb of Horrors" seems to lead this category.
"Masks of Nyarlathotep" for Call of Cthulhu has a reputation that I found fully warranted when I ran it. There were infantry units in the Somme that had lower casualty rates than that party.
"Night Train" for Deadlands is a notorious total party killer.
Any others?