The Sword in the Stone is technically a Sword in an Anvil on a Stone. Early swordsmiths would shape a weapon using a smoothed flat piece of stone, marble for preference. The term for one was a "sword-stone". When iron anvils began to take over, the term stuck around for a while. So the Sword in the Stone was thrust through two Sword-stones, the first a "modern" one of iron, tho other an old pagan one of stone. The Sword in the Anvil isn't as poetic as the Sword in the Stone, so the name stuck even though the language has moved on.
The Three Holy Kings in Christianity are neither holy nor kings, and we're not sure if there were three of them.
The belief that there were three of them comes from the verse in the Bible that says that "wise men" (who were likely astrologers) brought gifts of gold, frankincense, and myrrh. Three gifts, so it was assumed that there were three "kings".
At least some interpretations states that they pointed out that He was a king (gold), God (frankincense) and sacrificial Lamb (myrrh - which was used in burning rituals).
Speaking of the Bible, there are four books of John. The first one is called the book of John. The second is called First John, and so on. The explanation? John is technically the Gospel of John, while 1 through 3 John are The [X] Epistles of John.
Early Christianity had "Virgins of the Church," women who dedicated their time and money to helping the religious community. While celibate, they were usually widowswhose children were grown, since these women were more likely to be well-off and had few other social responsibilities.
Heracles means "Glory of Hera". He's Zeus' son but not Hera's (he would be her nephew/stepson) and was given the name in an attempt to please her. It didn't work.
Although this may be because the myth of Heracles has been handed down to us in the Hera-phobic Theban version. It would appear that in the lost version from Argos, a city that took worship of Hera very seriously, the relationship between Heracles and Hera was portrayed on more friendly terms.
Another explanation: most of Heracles' greatest feats came from Hera's attempts to kill him, meaning that she is the unintentional source of his glory.
Epiphany (6 January), also called "Twelfth Night", is actually 13 days from Christmas Day (if both are included in the count). This is because the festive season, or "The Twelve Days of Christmas", traditionally began on Boxing Day. This confusion has in recent years caused some to erroneously identify January 5 as "Twelfth Night".
In the second-century Jewish compendium of legal rulings and oral traditions known as the Mishnah, there is a tractate called "Beitzah," meaning "egg." The tractate is about festivals and their traditions, and not, in general, about eggs. It's only called "egg" because the first word is "egg", because at the time eggs were a common festival meal.
This Jewish tradition of choosing the first major word in a work to name it, whether it actually applies to the entire work or not, is a lot older than the Mishnah. The Book of Exodus is called "Sh'mot" or "names." There's a lot that happens in Exodus (what with the liberation from slavery, the splitting of the Red Sea, and the giving of the Ten Commandments), but not much about names. But the first sentence is about the names of the people who went INTO Egypt. So Exodus is stuck with a non-indicative name to this day.
Parshas (sections of the Torah read each week) are like this too. Special mention goes to "Chayei Sarah," which means "the life of Sarah." It begins by announcing her death. ("The life of Sarah was 127 years...") Possibly Fridge Brilliance since it mainly deals with finding the next Matriarch, Rebecca, and thus concerns the legacy of Sarah's life.
The Books of Samuel are not an example of the "first word" principle mentioned above, but they fit this trope. While Samuel could be considered the protagonist for the first sixteen or so chapters, he actually dies before 1 Samuel finishes, and overall they concern the life of King David.
Some critics have observed that that the Christian Science sect founded by Mary Baker Eddy denies both the deity of Christ (a central tenet to most of Christianity) and the reality of matter (a central tenet to most modern science).
The name of the YoukaiTengu translates to "Heavenly Dog". Nothing about the Tengu is canine in design.
Although usually the Vestal Virgins were an aversion of this trope, there were occasionally exceptions. This happened when one of the Vestal Virgins died before her term of service (normally 30 years) was up, and someone was needed to replace her. In that case, a young widow or divorcee might be selected for the position if there weren't any actual virgins available. note (Which might have happened more than you'd think; originally, the position was only open to patrician girls, but many parents were reluctant to commit their daughters to 30 years of celibacy instead of marrying them off and hopefully getting a grandson out of the deal, so it was eventually opened to plebeian girls, and even eventually to the daughters of former slaves.) So it is conceivable that there might have been a dearth of actual virgins available to fill a Vestal Virgin position at times, especially when one was needed on short notice.