According to a recent article from the Association for Psychological Science, just one minute in a threatening situation impairs memory. Researchers at the University of Portsmouth found that less than 60 seconds of all-out exertion can seriously impair their ability to remember details of the incident including police officers, witnesses, and crime victims.
The study found that during times of exertion, the brain pumps more blood and places more attention to those areas of our body that require it to survive. Thus, attention to detail and memory functions suffer as a result. One of the tests resulted in non-exerted officers being able to correctly identify a suspect twice as much as an exerted officer in the same situation.
Police officers are trained to articulate facts of a situation with specificity. Most incident reports are fact laden, heavy with descriptive words. All incident reports are completed after the fact and at the recollection of the officer involved. Over my years of experience, I have seen the same words describing situations over and over again. It's never "there was light odor of alcohol" or "he smelled like he had been drinking" or "the suspect had a bruise on his head." Instead it's "there was a strong odor of alcohol emanating from the suspect" or "there was bruising about the neck and head." I don't blame police officers for having these templates in place but this fact supports this articles assertion that there are problems recalling exact events when your adrenaline is pumping.
For these reasons, thank God for reasonable doubt.