Once I was told that I deserved to be fired and, another time, I was told that I am unable to think properly so I shouldn't work as a software developer.
Both remarks were quite painful because they were not questioning my ideas/opinions but my professional abilities. I confess that in my "down" moments those thoughts tend to pop up even years later.
Needless to say, in neither case the discussion originated from a technical issue, both times we were arguing online about politics.