Can a defendant be found mentally incompetent to stand trial?

"A person cannot be tried or adjudged to punishment while that person is mentally incompetent." ( 1367, subd. (a).) A defendant's trial while incompetent violates state law and federal due process guarantees. (Pate v. Robinson (1966) 383 U.S. 375, 385; People v. Pennington (1967) 66 Cal.3d 508, 516-517.) A defendant is mentally incompetent to stand trial if, "as a result of mental disorder or developmental disability, the defendant is unable to understand the nature of the criminal proceedings or to assist counsel in the conduct of a defense in a rational manner." ( 1367, subd. (a).)