This happens when there is more intense surveillance/ screening for the outcome among exposed than among unexposed.
Example: let's assume that the objective of a case-control study
is to assess if a history of past trauma is a risk factor for AS
(ankylosing spondylitis). Cases of AS are compared with a random sample
of the general population with regard to a history of past trauma.
Having a history of trauma, which increases the likelihood of having
X-ray investigations, will lead to a higher likelihood of diagnosis of
AS in persons with this trauma history than in the general
population. Therefore the proportion of AS cases with a history of past
trauma is higher among cases, and the related odds ratio (OR) will be overestimated.