In a way, nothing is out of bounds. It's very dependent on the situation, setting, frame, and follow-up of the person doing the flirting. Real complex. The boundaries are incredibly vague, it's an utter minefield.
There are some good guidelines out there though, I guess.
If you pay attention to how your actions factor into the process of making/allowing the person to feel comfortable with you, then that will help. Feeling uncomfortable is much of what prompts a recipient to shoot a flirting person down, while feeling comfortable is what generally prompts the recipient to allow it to progress.
Consider that women (as I understand it?) have a highly keen wariness for the potential stalker/rapist, and for good reason. That first link is a good read, btw - controversial but good.
It's a hypersensitivity that guards against, frankly, the vast majority of ways a guy could approach a woman. So basically, you cannot effectively flirt with a lot of women unless she feels okay enough to bring that wariness down. That said, it's not an off/on sort of thing, I don't think. So while she at one point she may be more receptive, it still doesn't quite mean you can say anything. So, taking the above Saturday Night Live skit as an example, the difference between the two guys wasn't entirely their first-impression attractiveness, it was also because the less attractive guy could not lower the woman's wariness(not necessarily because of his looks, but possibly).
I know, it's just a stupid skit, but I'm simply trying to make a convenient, if imperfect, illustration.
So, anyways, at the moment I can't think of something specific, but perhaps it's at least a useful perspective to have on how or when you can avoid crashing. From this perspective, the focus is not on what you say, but how you are perceived, if/when that perception changes, and what range of options exist at a given point.
But, ladies, please correct me if I'm wrong on any of this.