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 Advanced Energy Industries, Inc. (NASDAQ:AEIS) fell 21% Tuesday after the power conversion technology specialist released mixed first-quarter results and disappointing forward guidance.

So what: Quarterly revenue rose 26% year over year to $140.9 million, which translated to adjusted net income of $18.1 million, or $0.43 per diluted share. Analysts, on average, were looking for lower adjusted earnings of $0.41 per share on higher sales of $142.6 million.

However, Advanced Energy also expects second-quarter sales of $135 million to $145 million, with adjusted earnings per share of $0.34 to $0.40. By contrast, analysts were modeling Q2 earnings of $0.47 per share on sales of $154.48 million.

Now what: Advanced Energy blamed its weak outlook on a decline in OEM customers' second-quarter bookings, which was "consistent with industry reports." To its credit, Advanced Energy also suggested that its own diversified product portfolio helped to balance the negative effects of those declines.

In any case, it appears much of investors' pessimism might be priced in, with shares now trading below nine times next year's expected earnings. Even if those estimates drift lower as analysts have time to fully digest today's news, I think today's plunge might just offer a great entry point for long-term investors to add to or open a small position.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.