One myth from I always heard was:
"If you open a present on Christmas Eve, there's going to be one less present for you under the tree on Christmas morning". That one has always scared me, so I always wait until the morning. I passed that one on to my daughter too, but she's somehow figured a way around it. hmmmmm

Another myth I always heard:
"Christmas sure came early this year!". THat one has always puzzled me, because I know that December 25th is always at the same time each year.....

And lastly:
"Santa might put coal in your stocking". Now, this has never happened to me, so I don;t know if it's really a myth or if I've just always been good.