When I was younger, and in my more vulnerable state, I was often susceptible to the instructions of other girls in my bunk at camp as to what was proper halacha. Afraid not to be as "frum" as they seemed to be, I followed their instructions, too scared to ask any questions lest I appear less knowledgeable than they. As I grew older, and wiser, it came to my attention that most of what they had told me were . . . well, "total and complete fabrications" is a little strong (and often not really applicable - some of them are just halacha that is now defunct), so I term these "Bais Yaakov Myths."
So I present, for your edification, the short list:
1. You can't daven in your bedroom.
2. You can't drink water that's been uncovered overnight.
These are the two most common ones that I remember being swayed by. What myths have known you?