It is definitely possible. Not very common, of course, but definitely possible. I had my tubes tied when my second son was born via c-section in 2000. At that time I was sterilized by having my tubes tied using a modified Pomeroy technique. Basically that means a section of tube was removed and the ends were burned shut. Four years later, in March of 2004, I found out I was pregnant. The result of that was my third son who was born on October 16, 2004. My OB/GYN told me at the time that more often than not, pregnancies as the result of a tubal ligation failure are viable pregnancies. Ectopic pregnancies do happen as well, as my best friend found out a few years ago. She also got pregnant as the result of a tubal ligation failure (she was sterilized using clips). Her baby was in her fallopian tube though and she had to have emergency surgery to remove it.
So, like I said, it's definitely possible. It's still the most reliable form of birth control, but the only thing that's 100% is abstinence.