I know when I was starting out my career I had no idea what good manners were. One time at a milestone review I simply said: "I had to rewrite X's first pass he was using some older tech and there's better ways of doing it now." and it really blew up. My boss took me in his office afterwards and told me that probably wasn't the right thing to say that I had in fact insulted the other guy and he was hurt. Believe me it was never my intention to hurt anyone I was just stating what I did and what I knew. I apologized and we got along afterwards no harm in the long run.
That incident taught me to always be wary of corrections or talking about someone else's work. They put their heart and soul into what they do and I should respect their code and them.