Why does Matthew quote Zechariah but attributes it to Jeremiah?

Answer:

"Then that which was spoken through Jeremiah the prophet was fulfilled: "And they took the thirty pieces of silver, the price of the one whose price had been set by the sons of Israel; and they gave them for the potter's field, as the Lord directed me" (Matthew 27:9-10).

"I said to them, 'If it is good in your sight, give me my wages; but if not, never mind!' So they weighed out thirty shekels of silver as my wages. Then the LORD said to me, 'Throw it to the potter, that magnificent price at which I was valued by them.' So I took the thirty shekels of silver and threw them to the potter in the house of the LORD" (Zechariah 11:12-13).

There are a number of possible answers:

1. A few wonder if the reference to Jeremiah was accidentally inserted into the text. That sometimes happens when copyists accidentally pick up someone's margin notes. The difficulty here is that it is odd that the margin note was wrong. Also, only a few later manuscripts are missing the attribute to Jeremiah. The missing attribution is more easily explained that later copists tried to "fix" the manuscript by leaving out what appears to be an errant attribution.
2. A similar suggestion is that sometimes manuscripts contain abbreviations to save space. Jeremiah would be abbreviated in Greek as Iriou and Zechariah would be abbreviated as Zriou. An accidental one-letter change would create a large difference in the reading. The problem is that abbreviations aren't used in all manuscripts and the early manuscripts all have Jeremiah.
3. Another suggestion is that since the section of the prophets in the Hebrew Bible starts with Jeremiah that Matthew is using the name of the first book to represent all the prophets, but this seems to be a stretch to me as you can't find evidence of this being commonly done.
4. Yet another explanation is to note that if Matthew is quoting Zechariah, it is a very loose translation of the text. The phrase "gave them for the potter's field" cannot be derived from "threw them to the potter." Jeremiah does have statements regarding the purchasing of a field (Jeremiah 32:9-12), though the purchase price of that field was 17 shekels. It is possible that Matthew is quoting something that Jeremiah said but was not recorded and that Zechariah also said with a variation. The difficulty with this explanation is that an unrecorded statement by Jeremiah would not serve as proof for which Matthew is using it.
5. Related to the previous point, since Jeremiah has prophecies about potters (Jeremiah 18 and 19), spilling innocent blood (Jeremiah 19:4), the purchasing of a field (Jeremiah 32:9-12), and that where Jeremiah broke a jar would be used as a cemetery (Jeremiah 19:11). It is possible that Matthew is saying the events of Judas' death were both hinted at by Jeremiah and more plainly stated in the quote, that loosely comes from Zechariah that speak of the price and where the money was returned.

This is one in which the various offered explanations are not fully satisfying. Those who say this quote should be attributed to Zechariah have to explain why Matthew doesn't really quote the passage. Yet, at the same time, we can only point to some of the ideas and not an actual saying in Jeremiah's writings.