Then, I discussed with someone on the channel of a hackspace I used to go to (but had just started attending so I didn'n know the people well); I had never seen the guy IRL but he said he would attend the next session so I could figure out who he was in real life.
But when I come, none of the participants were him. And then a female participant shows up and a while later it turns out that "he" is her, and I was shocked to see how unexpected this felt.
I'm not implying that the IRC user being female was in any way shocking, nor did I believe that the women who had joined us (and, this time, the only female participant around) could not be a "real" participant to a hackspace. It's just that, while I had wondered of every participant whether it was that IRC guy or not, I just hadn't made the connection for her.
And, thus, I realized that, without even noticing it, no matter my opinions about gender prejudices, I must have had a pretty strong mental image of that IRC user being male, for me to be so surprised when she turned out to be female. IRC wasn't gender-neutral. It was male by default.
I'm not sure how related to the discussion this is, but for me this was the moment when I realized that prejudices weren't just something that stupid people did, and that I was also influenced by them even if (especially if) I didn't notice.