Ex. 23.19 (also Ex. 34.26, Dt. 14.21) gives a seemingly strange prohibition: “You shall not boil a young goat in its mother’s milk.” (ESV). So observant Jews today do not eat cheeseburgers or pepperoni pizza in case the cheese and beef are some how related. What did this ancient ban mean?
One consideration should be pointed out when reading the Old Testament text: the Hebrew was written with consonants only and the vowels were supplied through the reader’s familiarity of the language. It has been pointed out that the words for “fat” and “milk” are identical in their form (consonants). So it may be related to the prohibition against eating fat (Lev. 3.17, 7.23). While this may be a possibility, the Karaite Jews insist that they have correctly maintained the vowel marking as “milk” and not “fat.” in this particular prohibition.
Sometime in history a certain Jewish sect (Karaites) which placed more authority upon the scriptures rather than the “oral law” of the Rabbinate Jews (the dominant sect of Judaism) added vowel marking under the consonants to preserve the language after Hebrew was no longer a living language spoken by large numbers of people. One reason for the demise of Hebrew (though it was preserved to a degree by the religious professionals of that faith) was the dispersion of Jews both after the Temple destruction of 70 CE and particularly the scattering after The Bar Kokhba revolt (132-135 CE). So this particular sect maintained the bible (Old Testament) with the letters (consonants) and the vowel marks under the letters preserved the distinct words (in cases where a question could arise which word was meant as some were identical).
These Karaite Jews maintained what is known as the Masoretic Text. The earliest examples date from about 1000 CE. This is the standard text accepted by Christians and Jews. When the Dead Sea Scrolls were discovered, there was found remarkable agreement with this particular source. Also, The Septuagint (the Greek translation from about 250 BCE of the Hebrew Bible and which the Christians of the 1st Century used) in very large part also finds agreement with The Masoretic Text. So, if the preservation was essentially accurate, what does the command refer to which speaks against eating meat and milk together (or more properly: “boiling a young goat in its mother’s milk”).
Another, and better, explanation may have come from an archeological excavation at Ugarit uncovering a Ugaritic text which details a fertility ritual where a kid is cooked in milk and the mixture is poured on the fields. So, if this is indeed an ancient Pagan practice, a better rationale is seen for the prohibition concerning this odd practice. The Jews were not allowed to mimic these false beliefs but to rather trust in the True and Living God.
Source: https://en.wikipedia.org/wiki/Karaite_Judaism#Karaites.2C_Aharon_Ben_Mosheh_Ben_Asher.2C_and_the_Masoretic_text (see under Kosher laws).