Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
In the last sentence, it would be an error to say This set of
instructions are not complete. The subject is the singular set; the
plural word instructions is merely part of a phrase that modifies
the subject.
Similarly,
The inclusion of intervening words between subject and verb often
causes confusion.
An extensive collection of measures~ maintained in the laboratory.
Engaging in dangerous contact sports~ not for the faint of heart.
The shape of the eyebrows~ sometimes affected by facial expression.
An assortment of ribbons and bows was lying in the drawer.
One thing after another has conspired to make me late.
Note: Errors attributable to this type of sentence structure seem
restricted to when the subject is singular and the phrase contains
a plural noun. When it's the other way around, the effect of the
intervening words is less distracting. Few people would say The
pupils in her class is extraordinarily well behaved or The inscriptions
on the tombstone was almost worn away.
Exceptions
• In some cases, the subject and its modifier could be interchanged,
with little effect other than a slight change in emphasis:
The committee has come up with a set of guidelines that makes the
process easier to follow.
The committee has come up with a set of guidelines that make the
process easier to follow.
Both these constructions are grammatically acceptable; the difference
in nuance is that in the first case the guidelines seem to be
acting as a unit, and in the second case to be acting independently.
• The phrase a number of is always treated as a plural, even
though a number looks like a singular entity.
210
A number of vouchers have already been handed out.