1. Dresses etc, never fitting perfectly "up top"
It sucks to find the perfect dress that fits you amazingly, except for in the boobage area. There, it will slouch, or look all wrinkly and sad, like a deflated balloon. It will basically scream at you, "LOOK AT WHAT YOU DON'T HAVE!" So you have to stick with oversized looking dresses in general if you don't want to sport the slouchy look.
2. Wearing a sports bra, bikini or swimsuit (no padding)
With the magic of most bras, even those who only have minimal padding, at least you look somewhat decent. But with sports bras, bikinis and swimsuits? Good bye almost-existent boobs, you will be squished to death.
Ah, yes. I know they're flat. You know they're flat. Can't we just leave it at that? Of course not. Because there's always some douchenozzle that feels the need to point out just how ridiculously flat you are, how it's funny to them, how it makes you unfeminine and undesirable, etc. I get it. You don't like small boobs. I'm over it, so why don't you get over it. I know exactly what I look like, I do in fact own a mirror, so when you tell me just how nonexistent my boobs are, you're not exactly informing me of any shocking news.
4. People assuming you haven't hit puberty yet
Yes, hello, I'm just here to inform you that puberty doesn't always = more boobs. Yes, puberty is indeed a very magical period during which many things can happen, but not for everyone.
5. People also assuming that you will have "the butt to make up for it"
Like sometimes when I mention my flat-chestedness here, some guys have commented "hey, look on the bright side! At least you surely must have the butt to make up for it!"
... In what world does that even make sense? Like it's some kind of weird math problem, minus boobs equals plus butt? No. Nono. Of course there are women who are more of a pear shape but just because you have small boobs it doesn't automatically mean you have a bigger butt, especially if you're just thin in general/all over (like me). Just stop.
6. Girls with bigger breasts who complain about their own boobs being "too small"
I truly feel so sorry for you.
Despite all these things that I have encountered and heard as a girl with a flat chest, I still love my body and I still love my small boobies. I don't care if people think I'm less feminine, or not attractive enough. My boobies are great. Besides, I'm hella awesome anyway. I don't need boobs to prove that.