Consider C-Com (CMI), a developer of mobile satellite technologies. In early April, the company announced a special, one-time dividend of 2 cents per share for shareholders who held the stock on April 6th. A month later, quarterly results for the period ending Feb 28th came out. Nowhere in either the financial statements or management's discussion & analysis was this dividend mentioned...even in the section titled "Subsequent Events"! The dividend payment had already taken place, but shareholders relying only on the most recent financial statements would think the company's cash balance is much higher than it really is!
While 2 cents per share is not a lot for most companies, it represents about 7% of this company's market cap. Furthermore, it is not difficult to envision this situation occurring for other companies that hand out larger, even more material one-time dividends.
When you're interested in investing in a company, read all the recent filings, not just the ones that came out following the last quarterly report. There may just be events in the closet that are material to your valuation.