I'm pretty sure that I have them all the time because I sometimes wake up with strange images in my head. But these images disappear very quickly and I can't tell someone what I was dreaming about even minutes after waking.

Experiment: Bring a pen and paper to your bed, and when you wake up, the first thing you do (seriously the first; a minute of delay can make a huge difference) write what you remember. If you don't remember the beginning, just quickly start writing from the part you remember. If any idea comes to your head during writing, just make a small note (two-three words) and continue writing. Do this every day, at least 5 days in sequence.

Why do I suggest this? My case may be different, but after I wake up and think about something else, I usually forget what my dream was, even forget that I had a dream at all. I would swear that I rarely dream, but when I did this experiment, I had a dream every night (and if I woke up many times during the night, there was a different dream each time). Without the writing I wouldn't even notice. Even the written record seems suspicious -- I read about a dream, and I remember "yeah, I had a dream like this maybe a month ago", then I look at the date and see it was yesterday! So my experience is that my memory is absolutely unreliable in this area. Also, this may be a coincidence, but when I remember a dream, it is usually a bad dream, because it makes me think about it when I wake up.

EDIT: Now I realized a similar experiment with rationalization could be useful. :D