Take a look at the sentence again:
Josie, Don's Cairn terrier, will bark at anything that
moves, squirrels, wind blown leaves, passing
cars, and her own shadow will start her yapping.
The problem occurs between moves and squirrels. Josie, Don's Cairn terrier, will bark at anything is a complete sentence. Wind blown leaves, passing cars, and her own shadow will start her yapping is another complete sentence. Because these two parts have just a comma between them, they create a comma splice, a major sentence error.
To fix the problem, you could put a semicolon between moves and squirrels.