The problem with this is that proving that someone didn't forget something beyond a reasonable doubt is basically impossible. People forget stuff all the time. Especially once you throw in the stress and change in environment of a legal proceeding.

The result is that either the law has no practical effect because anybody can claim they forgot, or the courts fudge the requirement to prove that beyond a reasonable doubt in order to give the law effect, and then you put innocent people behind bars because they really did forget.