Et tu Brute? How wrong I was! (6,4,5)
I believe the answer is:
famous last words
This clue is a double definition.
'et tu brute?' is the first definition.
(Julius Caesar's last words in Shakespeare)
'how wrong i was' is the second definition.
(phrase used to show an earlier statement was wrong)
(Other definitions for famous last words that I've seen before include "Passing comment" , "Ill-judged comment" .)