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 SanDisk (NASDAQ: SNDK), a leading supplier of NAND flash memory, crashed on Thursday after the company revised its previous revenue guidance lower. As of noon, the stock was down more than 16%.

So what: SanDisk now expects first-quarter revenue to come in at roughly $1.3 billion, down from a previously guided range of $1.4 billion to $1.45 billion. The company's press release explained "The change in first quarter revenue estimate is primarily due to certain product qualification delays, lower than expected sales of enterprise products and lower pricing in some areas of the business. The Company expects continued impact to its 2015 financial results from these factors as well as the previously identified supply challenges, and now forecasts 2015 revenue to be lower than the previous guidance."

In addition to guiding its first-quarter revenue lower, SanDisk also withdrew all other forecasts for the first quarter and the full year. The company will provide an update during its earnings conference call, scheduled for April 15.

Now what: Demand for flash memory can be volatile, and this isn't the first time in recent history SanDisk's performance has taken a hit. In 2012, the company's revenue dropped by more than 10%, and profits were more than cut in half compared to the previous year.

This lowered guidance may also cause margins to contract in 2015 compared to previous years. SanDisk's operating margin has been erratic over the past decade, ranging from 7%-30%, excluding 2008, when the financial crisis caused big losses. In 2014, SanDisk's operating margin was 23.5%, and I suspect that when new guidance is announced it will call for lower profits in 2015.

This isn't the end of the world for SanDisk, but investors should remember that valuing a stock based on results during a great year, like 2013 or 2014 for SanDisk, can lead to disappointment when that performance isn't maintained.