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 DexCom (Nasdaq: DXCM), which is developing an ambulatory glucose monitor for diabetes sufferers, closed down more than 18% after giving full-year guidance that may have disappointed investors. (We'’ve seen it happen before.)

So what: Second-quarter results appear to have come in above estimates, though it's difficult to tell exactly what the Street was looking for. DexCom booked an $0.11-per-share loss on $21.4 million in overall revenue, $15.2 million of which was related product sales. Analysts were expecting a $0.12-per-share loss and $20.8 million in revenue. Whether they were counting all revenue or just product revenue isn’t clear, though I suspect the latter.

Now what: Why? In reiterating guidance during yesterday's call with analysts, management called for $67.5 to $72.5 million in just product revenue for the full year. Wall Street wanted $78.1 million of … some sort of revenue. Regardless, judging by the size of the selloff, this looks like a miss. Do you agree? Disagree? Weigh in using the comments box below.

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