What is a financial restatement?
A restatement is when a company reports a different value for a figure it has already published. Some are announced — in an amended filing, or an 8-K telling investors the original statements should no longer be relied upon — but most changes arrive quietly inside a routine later filing.
The word covers a wide range. At one end sits a correction of an error material enough that management formally withdraws reliance on the original financials. At the other sits a comparative figure restated because an accounting standard changed, with nobody at fault and nothing concealed. Treating those as the same event is how a data point becomes an accusation.
For anyone working with historical data, the important consequence is mechanical rather than moral: a figure that changed after publication means the number in a current database is not the number the market saw at the time. Any study that joins on today's value is quietly using a fact that did not exist yet.
Answering how often this happens requires every vintage of every figure since the beginning of electronic filing, not just the announcements — which is why the question is usually answered with anecdotes instead of a rate.