There is always a chance that paint will end up where you don't want it.
First off, there's a wide variety of what passes as a paint booth. Lot of folks simply take a cardboard box and cut away one or more sides. For these, paint will almost surely end up outside of the booth.
There are also a number of booths that come with installed fans to draw the fumes into the booth. Some people make them themselves, while some people (like myself) purchase something designed for the purpose. Like Matt says, they do a pretty good job, but note that he says "most."
In addition to the filters, you'll want to limit the amount of paint that goes into the thing. I've got an Artograph 1520, and I wouldn't dare use a rattle can in it because they simply put out too much paint. I've also observed that if I spray with my airbrush wide open, I'll occasionally get some clouds of paint coming back out, though it will usually will end up as dust that can easily be wiped away.
I've had a few close calls where the brush went off when I didn't expect, usually during the paint dilution or cleaning steps.
How much or how little paint ends up outside the booth will really depend on your skills and how much you pay attention to your actions. Having a booth and keeping all the paint inside can certainly be done. It also helps if you have an area where mistakes can be made... I don't (my apartment is carpeted and the walls are wallpapered, so I'm pretty careful).
Other steps you can take to minimize damage...
1. Get some floor covering (i.e. one of those rubber mats they put under office chairs). Wall coverings may help too, but I don't think it's required.
2. I do paint mixing, dilution, etc. on an old cookie sheet (with edges). That way, if I knock over the bottle of thinner or paint, it will be contained (hopefully).
2a. Work with small quantities. If you buy paint thinner in quart-sized jugs, pour some of it into a smaller jar, and then work with that.
3. Keep your work area as clutter-free as possible.
Hope this helps.