In May 2003, Pieper was target of an attack by a confused deranged man who broke into his house in Aerdenhout, and stabbed his wife who made a full recovery. Pieper publicly blamed the Dutch police as the attacker was caught trespassing on the property, including with video proof, on previous occasions. The Dutch police refused to act until the attack almost killed Pieper's wife. The man who stabbed Pieper's wife was convicted and sentenced to compulsory psychiatric treatment in a closed judicial institute (Terbeschikkingstelling in the Dutch language). He then was granted a temporary leave in 2007. The man has since been reinstated into supervised custody.