If
one made any deviation from the rules of a Get, such as, being in
Babylon, dated it according to the calendar of Rome or Greece, or even
dated it from the construction or destruction of the Temple, or
incorrectly named the city of writing, the Get is invalid.
According
to Rabbi Meir, the woman needs to leave her new husband if she
re-married, but can not return to her previous husband. She also looses
her Ketubah payment from both, and many other benefits.
However, prevailing ruling is that she does need another Get but is not penalized as above.