Here is the passage again:
Maria was not watching her plate of barbecue very carefully. So Santana, the family beagle, snatched a chicken leg hanging over the edge. As baked beans and potato salad slid onto Maria's new sandals.

The third part is a subordinate clause fragment. This part has a subject, beans and salad, and a verb, slid, but the subordinate conjunction as keeps it from making a complete thought.

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