For me, it's whenever I'm treated differently simply because I'm female.
The time at school, when the boys were going to be in individual tents, but the girls would be in a large shared tent. (I ended up with my own tent.)
The party I went to, where, in the morning, the father told the girls to do the washing up, and the boys to take down the tent. (I insisted on taking down the tent.)
At work, when I answered a colleague's phone, as he was waiting for a call from a support engineer, and the person on the other end assumed I was a secretary.
At work, when the signs on the door says, "Beware of men working behind doors," as if it's suddenly no hazard if I am the one lifting floor tiles and moving trollies of hardware around.
At work, when a male colleague said, "You can think logically, because you don't have enough hormones."
At work, when I look round, and don't see any other women in a department of 40.
At work, when I have had to fight for a payrise and promotion, even though I outperform most of my male colleagues. (Still working on the promotion.)
These are the main things, but there have been many, many other ones over the years.