A feature introduced in Mac OSX 10.9 (Mavericks) which remains in OSX 10.10 (Yosemite) performs validation of ARP requests, which results in unicast ARP requests. These requests will time-out if the gateway on the host does not receive a timely response
User Account Control (UAC) can sometimes be a pain in the neck which is why I have looked for ways around scripting in customer environments that have UAC enabled. This article contains solutions for this issue.