If the seller claimed that the investment is low risk or does not explain that the principal could be lost if there is a "credit event".
Low risk means
1) 5% chance of losing 10% capital, or
2) 0.00005% chance of losing 100% capital.
Unless the seller already had insider information, nobody will expected Lehman to collapse just like that.
Everything is in the prospectus.
Assuming you only received the propectus immediately after you signed up.
If you do not sue for misrepresentation within reasonable time upon receving the prospectus, the court will take it that you have accepted the deal willingly. Many Lehman investors were earning interests, and happy with the product for a period of time.