I once went to my dentist because I had tooth pain. He said nothing was wrong and shooed me away. I came back several months later and he was astounded at how big a cavity I had gotten, they actually had to put a spacer (metal tube looking thing) in it because there was such a big hole. Oddly enough, this was the same tooth that hurt earlier.
Another time, I felt a hole in my tooth, this worried me and I went to see him again. He said there was nothing wrong. The dental hygenist came in and looked at me and told him I had a hole in my tooth. I then got a filling which sticks out about 2mm past my tooth. That greatly annoys me.
I had tooth pain yet again, they said I had a cavity and filled it. Tooth pain then continued. They said it was my wisdom teeth and I had to go through surgery to get all 4 of them out. Tooth pain continued, they said 'woops, turns out your tooth is dying, we're just going to have to give you a root canal.'
After root canals, they have to put a crown on the tooth, so they drill down the outside of your tooth into a tiny nubbin and cover it in coating to make a temperary crown, take molds, and send away for a real crown which is porclin covering metal. When drilling my tooth, it hurt very, very badly. I was crying because of it. They wouldn't pause. When they got the real crown, they put it in and told me to bite down to get it in place and see how it feels. CRACK! Two more weeks to get a new crown. They told me to bite down to get it in place and see how it feels. CRACK! Two more weeks. They completely filed down my crown so it lacks any sort of groove whatsoever. This is a molar, it's supposed to be nice and ridgy, having a completely smooth mound in my mouth posing as a tooth drives me nuts. Not to mention that because they filed it down so much, my tooth then dropped lower to fill in the gap.
Edit: The reason I kept going to the guy was because my mother worked there. Free dental cancles out crappy dental.