I've done a little playing around with Starry Nights (the software
) and checked back to my date of birth, etc .... despite the fact that my birthday falls very squarely in the middle of the Cancer period when I was born (mid July) the sun was most definitely in Gemini. Many people will find that a similar thing will apply, generally speaking if the date you were born lies within the first half of your sign the sun was probably REALLY sitting within the sign previous to yours.
If you want a hint as to why this inaccuracy has occurred set your favourite "sky" software to whenever your birthday falls prior to October 1582 and check the suns position against the constellations, then check it again at your following birthday (post october 1582).
Side note: In addition to those who were actually born under the sign of Ophiuchus (g'day Ron!) I have noticed that it may be possible to have been born under the sign of Orion!