There was no mistake. The brain swap was completely intentional.

"JFK lost half his brain mass as a result of the shooting, according to FBI agent Paul O'Neil. Floyd Riebe, autopsy witness,said, "it was less than half a brain there." 

See the front head shot here, the hard evidence has been there for fifty years: Eyes Wide Shut. Zapruder Film Slow Motion (HIGHER QUALITY) - YouTube

Remember, the murderers of JFK were running the non-investigation into his death.

A normal brain weight, in average, weighs 1300-1400 grams"

Web link: JFK Lancer

Amazon review comments: debate on JFK assassination