Then we hurried, only a few hours of darkness remained and our 
   brother lay as yet unburied.  The ceremony was brief but solemn.

                      -- Excerpt from the diary of Helga Hufflepuff

They lied to the students about Salazar's disappearance and imprisoned
Rowena's failed experiment in his cavern.  Helga wasn't even permitted
to tell their son, but warded her diary against opening for a thousand
years and left it to him. 

The accident could well have doomed the school in its infancy, but by
covering it up Godric Gryffindor turned his convictions into lies.
Lies I suspect he regretted to the grave.