Recently one of my customer reported error "APP-FND-01564. Oracle Error 1403 in fdxwho' while checking record history.Initially I was like what is this all about.
On Further Investigation I found that If last_updated_by/created_by column (column to capture the user_id) has value that doen't exists in FND_USER table , then if we go to Help>record History , Oracle Apps will throw this error.
If you ask me , I say nothing much to worry about.But if you want update this column with a valid value only if it is a custom table .For Oracle Standard ERP table , it is always good to contact Oracle Support.
set last_updated_by /created_by = Valid value
where xxx = Y