Review the sentence:
Emma Sue was delighted when Professor Nguyen returned her perfect calculus
test with an ear-to-ear grin.
With an ear-to-ear grin, a prepositional phrase, is describing test when its target should be Professor Nguyen. When a modifier is next to a word it doesn't logically describe, it is misplaced.
To fix the problem, you could organize the sentence like this: With an ear-to-ear grin, Professor Nguyen returned the perfect calculus test, which delighted Emma Sue.