In the Samoan culture dreaming about your teeth falling out is a sign that someone in your family/someone you know (depends on where on the island you ask) is going to pass away. Of course this is the same culture that doesn't sweep the floor at night or brush their hair in front of a mirror after dark. The traditional beliefs are the most entertaining.
I've always had weird teeth dreams whenever I had a loose tooth. I spit out a tooth and end up having an impossible number of teeth coming out of my mouth. I swear, sometimes I could FEEL all those teeth coming out. *shudder* Luckily all my baby teeth are gone, so no more weird teeth dreams!
I've heard tell that teeth falling out represents financial problems. But I could be wrong on that.
But yeah, those dreams are awful. And they'd just happen to me in any sort of dream... like I could be just driving somewhere, randomly reach up into my mouth... BAM. Pull a tooth out. I could be banging this cute girl... push my tongue against a tooth... BAM. Just pushed my tooth out.
I had a funny dream where I realised it was a dream, and I could control it. It was actually pretty cool since it involved time travel- I needed the help of this lady from the 60's, and I brought her to 2012 to prove I was really from the future. Except 2012 had buses filled with giant bingo balls