I voted Hackers and now see that the majority of people have also. I've seen more problems with hacking than any other way. Having a high security level makes it a challenge for hackers and they CAN find a way in.
Few organizations, especially small to medium ones take internal security seriously. A lack of strong password standards enforcement, securing wireless equipment, inexperienced sysadmins, etc. all contribute to making this the most blatant and dangerous security threat to businesses today.