Fox found plenty to fret about. He pointed out that second-quarter cash flow, prior to share repurchases, slid from $55 million to $4 million. He also noted that total debt had jumped both sequentially and year over year.

But he dwelled on the company's sales trends in particular. Over the past two years, he noted, the company's retail sales comps have slid into negative territory. Meanwhile, he added, the company's commercial comps are increasing at less than half their old rate.

AutoZone's latest results looked particularly weak when compared to the surging growth recently posted by competitors.

"Recall that O'Reilly ( ORLY) and Advance Auto Parts ( AAP) reported positive, high-single-digit comps for their most recent quarters," Fox reminded readers. "We believe that AutoZone's emphasis on margins is causing it to lose market share."

Some analysts had banked on AutoZone riding the strong industry trend. But at least one hinted at a possible downgrade if AutoZone detoured from that path.

Whitaker Securities analyst Cid Wilson said he'd be focusing on sales to retail "do-it-yourselfers" -- which fell short of expectations -- when evaluating the company's prospects.

"If we see signs that AutoZone is participating in the upswing in DIY sales seen at other auto parts retailers, this could be a buying opportunity," wrote Wilson, who rated the stock outperform prior to Wednesday's report. "But if AutoZone and Advance see a divergence in sales, it could warrant a review in our rating and thesis."

AutoZone weathered at least three downgrades after it last disappointed analysts three months ago.

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