I get where you're going with that, but I would much rather someone tell me I hold a common misconception instead of saying my code is garbage. As much as the ideal is to remember that you are not your work, I still feel that my work reflects on me and my intelligence.

Giving someone an out is a great thing to do, especially for people for whom saving face is important. But I don't think "this code is garbage" does that. Something like, "that code is measuring timing incorrectly because X", conveys the same information without being inflammatory. "Garbage" is a value judgment, while just pointing out that it's doing something incorrectly is merely stating a fact.