Final answer:
The error in Brendan's sentence is the misuse of a semicolon with a conjunctive adverb. The correct sentence should have a semicolon before 'consequently' and a comma after it, thus reading: 'In 1989, a group of computer scientists created a computer program called Chinook; consequently, the program's algorithm finally became unbeatable in 2007.'
Step-by-step explanation:
The semicolon error in Brendan's sentence involves the incorrect use of a semicolon with a conjunctive adverb. The semicolon should be used to join two main clauses that are closely related in meaning and that are not joined by a coordinating conjunction. In the sentence, the semicolon should appear before the conjunctive adverb 'consequently', and the conjunctive adverb should be followed by a comma. Therefore, the sentence should read:
In 1989, a group of computer scientists created a computer program called Chinook; consequently, the program's algorithm finally became unbeatable in 2007.
This correction ensures that the sentence has the proper punctuation that adheres to the rules regarding the use of semicolons and conjunctive adverbs.