You have violated the primary key

Hi people of techyv!
Please allow me to share my experience here in techyv. While I was using Microsoft Access, I met a strange error. Please see below the error message.
Microsoft Access
You have violated the primary key.
As I read on the error, it makes me think twice, such a short error message "You have violated the primary key", that is why I have a hard time figuring out what went wrong. When I looked for same error message over the internet, none was posted as what I have. I am asking for some help on this error message. I am hoping to hear your ideas. Thanks.
