I made the mistake of assuming I wouldn't get in anywhere after a couple of bad grades freshman year and some "issues" that caused me to take time off on two seperate occasions. Then I made the mistake of not taking the MCAT seriously enough, choking during the test, and getting a 27, which is not in itself bad, but, when coupled with the fact that I withdrew on two occasions, and that my freshman year grades were not great, wasn't a good thing.
I actually wasn't even going to apply, but, then with some post bac work to pull up my grades a little, another MCAT prep course followed by an 11 point improvement, now I've gotten 8 interview invites thus far.
So, understand that stuff happens during your undergrad career, and that even if everything isn't perfect, this doesn't mean you have to give up. But it does help if you don't make all those mistakes to begin with.