Brachot 12 - The Prayers of the Kohanim
The Kohanim, the priests in the Temple, could say only those prayers which superseded their services. They therefore said one, not two, blessings before the Shema. Then they recited the ten commandments, and afterwards the Shema. Then they said the blessing after Shema, because it mentions trust in God. Out of the Amidah (Eighteen blessings) they said the prayer for the acceptance of their services, and then the three-part blessing for the people.
The Sages wanted to establish the reading of the Ten Commandments also outside the Temple, but soon abolished the idea, because it gave rise to the argument that only the Ten Commandments were true.
On Shabbat, the Kohanim added another blessing, and the outgoing watch would say to the incoming watch: "The One who causes His Name to dwell in this House - may He cause among you: love, brotherhood, peace, and friendship."
Every blessing should be said according the form that the Sages created for it. What if he started with one blessing in mind but ended with another? For example, he took a cup of beer and thought it was wine, and he started to say the blessing on the wine (which is more distinguished than that of beer), but then caught himself midway, and finished with the blessing on the beer? The Talmud tried to find an answer, but all examples are where even if he made a mistake, the blessing would still be valid, while in our case it would not be. Thus there is no definite proof, but for a simple answer - it is the conclusion of the blessing that counts.
Art: Pierre Auguste Renoir - Glass of Wine