Not all harm is obvious right away. The discovery rule is an exception to standard accrual: instead of starting the clock on the date of the underlying event, it starts on the date the harm was discovered, or reasonably should have been discovered.
This matters most in cases where injury develops slowly or is hidden from the claimant, such as medical malpractice, product defects, or fraud. Someone harmed by a mislabeled medication, for example, may not connect their symptoms to the product for months or years.
Courts apply the discovery rule narrowly. The claimant generally has to show they exercised reasonable diligence and genuinely could not have discovered the harm sooner. Simply not noticing is not enough on its own.
Because the discovery rule shifts when the clock starts rather than how long it runs, it can significantly extend the practical filing window without changing the statute's stated number of years.