People shouldn't be so sure that they have good LORs. You need to be very careful who you ask/trust, since some people can be pretty back-stabbey.
Last year, I was applying for a job, and had two people write LORs for me... an internship supervisor who absolutely adored me, and my thesis advisor. Both of them were all smiles, and told me they'd give me the best letter possible. And then, I got to the interview, and... got asked some very personal (and quite negative) questions, because of something "your... professor?" said in the LOR. The recommender had, instead of writing about me, wrote about my relationship with my family.
At first, I was paranoid that my super had done it, since he was a confidant for awhile. But then, later on, when I finished my senior thesis... I realized what happened. My advisor, all along, had been telling me good job, have fun, it's your project, wow nice find, this is excellent, etc... suddenly turned on me and said she was "greatly disturbed" by my final piece, even though the previous draft was the best thing since sliced bread (she even went as far to tell me some of my sources were crap, even though those were the sources she had personally recommended to me).
It turns out, she was only doing this as a power trip, that was the same reason she agreed to do my LOR. When I started asking around the dept about her, it turned out she was "volatile," into power trips, and would mess with grad students the same way. (My super, on the other hand, "couldn't hurt a fly.")
The lesson: Be careful who you take your LORs from. Don't desperately hunt for a prof to write one, and be willing to take one from someone less well known if you know they'll say better things about you. And don't be 100% sure that you'll have a good letter. Just don't.
That being said, I'm 75% sure that my LORs for law school are awesome. I was more careful this time, and I picked people I really trust.