Review the item:
My cat Buster loves to nap on warm appliances when
he sleeps on top of the microwave oven, his tail swipes the glass door like
a windshield wiper.
The problem occurs between appliances and when. My cat Buster loves to nap on warm appliances is a main clause, so it can stand alone as a complete sentence. When he sleeps on top of the microwave oven, his tail swipes the glass door ... begins the next complete sentence.
Because these two parts have only blank space between them, they make a fused sentence, a major sentence error.
To fix the problem, you could put a period after appliances and then capitalize the W for when.