A statute of limitations is a legal deadline for filing a claim. If the deadline has expired, the claim is dead, no matter how good it might have been.
Some statutes of limitations start automatically when a purchase is made, so you don't need to worry about starting them. However, others only start when a customer knows or has reason to know that he has a claim. To start these so-called "discovery" statutes, you should document the fact that the customer knows the bad news.
For example, if a customer calls about an article he read announcing that the issuer of some stock you recommended has filed for bankruptcy protection, you should at least put a telephone memorandum in the client's file.