Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Potbelly (NASDAQ:PBPB) plummeted 24% today after its preliminary Q2 results disappointed Wall Street.

So what: Potbelly shares have been crushed over the past year on rapidly slowing growth, and today's current-quarter outlook -- adjusted EPS of just $0.06 on revenue of $83.6 million versus the consensuses of $0.12 on revenue of $86.7 million -- coupled with downbeat full-year guidance only reinforces that trend. Management even warned that same-store sales would slip 1.6% for the quarter and be flat to down for the full year, suggesting that its competitive position and business model aren't strong enough to provide long-term value.

Now what: Management now sees full-year EPS of $0.18 to $0.21, well below its prior view of $0.43 to $0.46, on revenue of just $83.6 million. "Our neighborhood marketing approach will continue to be our primary focus in the near-term, but we will also take additional steps to drive comparable store sales growth," said Chairman and CEO Aylwin Lewis. "We have been encouraged by the strong customer reception and trial for our new Flats offering and believe we can continue to build this platform over time. Additionally, we intend to vigorously test a number of new marketing, menu and operational tactics during the second half of the year." More important, with Potbelly still boasting a rock-solid balance sheet and its shares now off more than 65% from their 52-week highs, the downside might be limited enough to bet a bit on that turnaround talk.