I still don't grasp how the comma in the embolded sentence below "is technically incorrect". It can be rewritten without the comma as
The contract allowed the defendant to deliver apples or oranges. Therefore the contract was not breached upon delivery of oranges.
3 There is one exception to the general rule that you should not separate a verb from its subject. If a comma is necessary to make the meaning clear, then you can insert one. For example, the comma in the sentence, ‘The contract allowed the defendant to deliver apples or oranges, and therefore was not breached upon delivery of oranges’, is technically incorrect. However, because the comma groups the term ‘apples or oranges’ visually, it helps the reader understand the meaning of the sentence and is therefore an exception to the general rule. Usually the ‘extra’ comma occurs (properly so) in sentences that have multiple conjunctions and need clarification.
Stacie Strong BA English literature (UC Davis 1986), MPW (USC 1990), JD (Duke 1994), PhD Law (Cambridge 2002), DPhil (Oxford 2003). How to Write Law Essays & Exams 5th Edition (2018). p 138.