Gross Anatomy

בכורות דף מה ע"א

מעשה בתלמידיו של ר' ישמעאל ששלקו זונה אחת שנתחייבה שריפה למלך בדקו ומצאו בה מאתים וחמשים ושנים אמר להם שמא באשה בדקתם שהוסיף לה הכתוב שני צירים ושני דלתות

Bechorot 45a 

There was once a prostitute who was sentenced to death by the king. Her cadaver was given over to the students of Ribbi Yishmael to corrode and perform osteological studies. After studying her bones, they concluded that there were 252 bones, and not 248 like the Mishnah in Massechet Ohalot (1:9) claims! When they reported this discrepancy to their teacher, he asked them, "Was it a woman which you dissected? A woman's pelvis does not ossify in the same manner that a man's does!"

It is worth noting that according to today's anatomists, the adult human only has 206 bones. A"A addresses this by quoting the Encylopedia HaIvrit, which explains that the Talmudic account of 248 was not meant to be an account of complete bones but rather of ossification centers. 

I would like to add a suggestion. Perhaps the difference lies in the definition of adulthood: anatomical adulthood is defined by the completion of ossification (which occurs at around 20 years of age) while Halachic adulthood is defined as 12 or 13 years of age (for females and males respectively) or the onset of pubic hair growth (if it occurs after that age). Anatomists would agree that at 13 years of age, there are somewhere between 270 and 206 bones in the body.