Probably when my then-girlfriend refusing to pass my demo to someone she knew at a local electronic music label (that it would have been completely appropriate for) as "it's not good enough". Looking back, she was completely right, my music then was nowhere near polished enough. Still, at the time I had a lot more faith in it and was a bit hurt by it.
I've rarely seen any bad criticism of my music, that couldn't be directly linked to some emotional reason to bias them against me/it. For example, one of person slated an album of mine with completely inaccurate claims/criticisms. Oddly enough, the very same chap had described a demo I released of a track not strong enough to make it onto the album as "some of the best music I have ever heard". The difference? At the time of the demo, he mistook it for something done by a famous artist. By the time the album had come out, we'd cleared up this confusion. Do you know what I really think? He felt like a tit for succumbing to emperor's new clothes syndrome and imagining a mediocre track was great, just because he thought it was by someone famous, so he lashed out at the album to somehow negate this. I just dismiss this sort of criticism out of hand and don't consider it at all. It works both ways of course, if people like you as a person, they might not be forthcoming about the things about it they think suck. A lot of my stuff has been done anonymously, precisely so I can see what people really think about it, without this pre-defined emotional bias from entering into it.