In the sentence: The boat, along with its crew, were lost at sea, what is the error?

The error in the sentence arises from subject/verb agreement. In this case, the subject of the sentence is "the boat," which is singular. The phrase "along with its crew" acts as additional information and does not change the number of the subject. Therefore, the correct verb form should be "was" instead of "were," aligning with the singular subject "boat." The sentence should read, "The boat, along with its crew, was lost at sea." This illustrates the importance of ensuring that the verb agrees in number with the main subject, and not influenced by intervening phrases.