Denial of Service Attacks
Primarily due to flaws in the Microsoft Operating System and an underlying paradox in the infrastructure of the internet, individuals or small groups are able to control tens, hundreds, even many thousands of unsuspecting computers and perform Denial of Service attacks against online targets.
Militaries posses weaponized DDOS systems and will use them in cyber warfare.
Organizations with an interest in dealing with this serious problem simply have not been able to.