In the scene when Billy is climbing back out of Sydney's window, she ask's him if he would settle for a PG-13 relationship and then flashes him. If you look at her shoulder, you can see her bra strap.
Ok. If you want to get REALLY techinical here. Sidney is what you call a "good girl". She probably has never even let Billy touch her under her clothes before -much less SEE under her clothes. And so for someone like that to be willing to open her shirt to expose her breasts EVEN with a bra...that's a pretty big deal. So who cares if she's wearing a bra! It could still be classified as a PG-13 sort of relationship.
Okay...This one, I must admit is an "open to your interpretation slip-up". For a second, I will say, that I was agreeing with those who said she just showed her bra. But then, I started to wonder. I figured the only way to find out the truth would be to whip out the original Scream script and see what Kevin Williamson's intention was. The excerpt reads as follows: "SIDNEY: Would you settle for a PG-13 relationship? BILLY: What's that?" and then "She pulls her flannel gown open for a split second...flashing her left breast. His mouth drops open...surprise, shock. Their eyes meet. They share a smile." Now, although the original script was written this way, the line about Sidney being a tease is not in there. It was added later. Hence, if the line was added in there, then what Sidney did could have been changed possibly to add more innocence and integrity to the character of Sidney. So, whether or not it is actually a slip-up is entirely up to the person viewing. I guess it's just one of those things that can go either way. Unless of course you want to go and ask the director himself. ::::::Looks around as though that may not be a bad idea::::
To Duh: Sid could not have "lifted" up her bra when she flashed Billy because she doesn't "lift" her shirt up. She opens it therefore he only saw her bra. It's impossible to open your shirt and lift your bra at the same time. Trust me. :)