Because it is event-driven, the 8-K is where surprises live. Quarterly results are furnished under it, as are auditor changes, restatement notices, material agreements and departures of senior officers.
Most must be filed within four business days. Setting an alert on a company's 8-K stream is the cheapest form of monitoring available, since the filing usually precedes the news coverage.
Example: Northwind Tools files an 8-K announcing its chief financial officer's resignation effective in six weeks, with no successor named. The shares fall 7% before any analyst note appears.
Related: earnings-report, restatement, edgar, form-10-q, pre-announcement