What's stopping them from claiming misfeasance or just 'unknown unknowns' in the face of malfeasance? Absent smoking gun emails where they laugh at how stupid t

What's stopping them from claiming misfeasance or just 'unknown unknowns' in the face of malfeasance? Absent smoking gun emails where they laugh at how stupid their clients are for buying the crap, how do you demonstrate this to a jury in a civil case?