I have now worked on my third court ordered criminal evaluation in which I could not decide whether the defendant was mentally ill, simply dishonest or wrongly accused. A variety of psychological conditions could cause a criminal defendant to incorrectly believe that he or she had not committed the crime at issue. The defendant may have been in a dissociative state, psychotic or deranged or delirious due to drug intoxication. In such cases, the defendant would honestly testify that he or she was innocent because the defendant has no working memory of the events at issue.
Alternatively, the criminal defendant might well know that he or she is responsible for the crime, but is attempting to dishonestly deny having committed the crime.
Finally, there is a third possibility: that the defendant is innocent and his or her denial is simply truthful.
The forensic evaluator often does not know whether the person being evaluated is actually guilty. People are, sometimes, falsely accused. Ironically, one would not have a legal basis to decide if the defendant is guilty until AFTER the trial and the verdict. But forensic evaluators generally do their work on competency to stand trial and insanity defense BEFORE the facts have been established by the court.
My solution has been to offer all three alternatives in my reports:
1. The defendant may have no memory of the crime due to mental illness.
2. The defendant is innocent of the crime, and that is why there is no memory of it.
3. The defendant is guilty and is simply lying about his or her culpability, as criminals tent to do.
Martin H. Williams, Ph.D. On Forensic Psychology 
This might be a typo. My solutions has been to offer all three alternatives in my reports:
3. as criminals “tend” to do.