Pirkei Avos 4:16

"Rabbi Judah would say: Be careful with your studies, for an error of learning is tantamount to a willful transgression."

Because Dan said so, here's my take (note that this is my own idea on this and may not really be reflected by commentators, and could easily be wrong):
If you're sitting and learning Torah, whether from some dumbed down artscroll book or hitting the Gemara, you have to apply yourself to your utmost. Why? Well if you're just a "regular guy" who knows you're Jewish but have no idea what halacha is, or even what the most basic transgressions are, and you eat meat and milk together or any number of other infractions you are an inadvertent sinner...
I think the Hebrew for this is shogaig.
Being an inadvertent sinner doesn't free you from all obligation to do teshuva, but it's not the same as being the guy who eats treif willingly...
All of the answers to how Hashem wishes us to act are in Torah and it is up to us to crack open a book and find out what's correct and what is right. But the exhortation to, "Be Careful" adds another layer to learning... because a failure when you've been given the answers takes one out of the "shogaig" category. To learn Torah incorrectly out of laziness is tantamount to purposefully transgressing, because an incorrect understanding of understanding of Torah will lead to incorrect behavior. So open the book and learn hard.