Upset failures - result in gate leakage
Catastrophic failures - occur in two forms, Direct and Latent
Latent failures occur when ESD weakens or wounds the component to the point where it will still function properly during testing, but over time the wounded component will cause poor system performance and eventually complete system failure. Because latent failures occur after final inspection or in the hands of your customer, the cost for repair is very high. Not only is this type of damage hard to find, but it severely affects the reputation of your company's product.
Upset or latent failures may pass your company's quality control testing program. In other words, static damage may occur that cannot be felt, seen, or detected through normal testing procedures.