Ordinary human reasoning may lead patients to provide an unreliable history of past experiences because of errors in comprehension, recall, evaluation and expression. Comprehension of a question may change depending on the definition of periods of time and prior questions. Recall fails through the loss of relevant information, the fabrication of misinformation and distracting cues. Evaluations may be mistaken because of the "halo effect" and a reluctance to change personal beliefs. Expression is influenced by social culture and the environment. These errors can also occur when patients report a history of present illness, but they tend to be more prominent with experiences that are more remote. An awareness of these specific human fallibilities might help clinicians avoid some errors when eliciting a patient's past medical history.