Identify the error in the underlined parts: (a) The project staff are (b) working on the weekend (c) so as not to delay (d) the project any further.

The word 'staff' is a collective noun. While in British English it can take a plural verb when emphasizing individual members, in standard usage and especially when the staff is considered as a single unit working on a project, the singular verb 'is' is preferred. The sentence 'The project staff are' should be 'The project staff is' since the staff is treated as a single body working collectively on the project. Therefore, the error is in part (a).