The number of times that people invoke "I don't recall" while giving testimony under oath says otherwise.
And for the record, a defendant claiming "they forgot" something within their control is valid circumstantial evidence of guilt or responsibility.
When claimed by the police or prosecution witness, it's just as much valid evidence against guilt, and has been used many times to get defendants off.
Basically, it's valid circumstantial evidence against the party/side that makes the claim of forgetfulness.