In Greek tradition if you have a bad dream and you tell someone before you eat anything your nightmare will come true.  However the same thing is not true if you have a good dream.  If you tell a good dream before eating in an attempt to make it come true, the gods will see through your trickery and it will not happen.

My roommate is half Greek and she learned this tradition from her mother.

This tradition is interesting because it reaffirms the power of spiritual beings as being above us.  This is humbling in a way and reinforces the idea that mere mortals should never try and outsmart the gods cause they will always be one step ahead.  The tradition is also interesting because it speaks to a very negative aspect of the culture, in this situation no matter what you do, it ends up with nothing good happening.

The tradition seems to also be related to the idea that if you have a wish and you tell it to someone it will not come true, like birthday wishes or wishes on a shooting star.