After starting to use the Cisco VPN client to do remote work on my McBook, I noticed that it would give an oh-so-useful "Error 51: Unable to communicate with the VPN subsystem" error fairly often, like this:

As it turns out the Cisco kernel extension is a bit flaky and craps out, so you have to restart it. Thankfully Anders Brownworth at anders.com worked out how to fix this inane Error 51 thing. Thanks Anders!