That had been unforgivable.
"Out Like a Light"
Gordon Randall Garrett
Sara's cheeks had grown red at this; but when the professor added,- "Between you and me, Molly, I think it's only fit for sick folks myself; but I suppose, as the saying is, we must eat by the card;" at which everybody laughed good-naturedly, her worried feeling wore off, and she began to think it would not, perhaps, be an unforgivable offence if one of them did commit a blunder or two.
"Sara, a Princess"
Fannie E. Newberry
It will be the one unforgivable thing, and I never forgive.
"The Box with the Broken Seals"
E. Phillips Oppenheim