After upgrading to OS X 10.9 (“Mavericks”), I started getting packet loss on our corporate network. This is apparently a bug in Mavericks relating to ARP, Unicast and GLBP:

https://discussions.apple.com/thread/5483424

A simple temporary workaround appears to work:

sudo sysctl -w net.link.ether.inet.arp_unicast_lim=0

For a permanent fix, place it in /etc/sysctl.conf:

$ grep unicast /etc/sysctl.conf
net.link.ether.inet.arp_unicast_lim=0

This is supposedly fixed in OS X 10.9.2.

Update 2014-03-16: seems to be better in 10.9.2, but not gone - I still need the above workaround on my system.