It was ruled on the field that he slid in before Irving Falu's tag, but Padres manager Bud Black challenged the call.
After replay review, it was determined the call would stand. Posey was stranded at second, however, as Thayer set down San Francisco's next three hitters.
The Giants went on to win, 5-3, after Santiago Casilla retired the Padres in the bottom half of the inning.
Will Laws is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.