If you're using Varnish cache, you will be familiar with the "service varnish restart" then the "oh, there's a config error and now all my sites are down!".
Fear no more: there's a command to check your Varnish syntax before you restart. Just run:
$ varnishd -Cf /etc/varnish/default.vcl
And it will tell you what's wrong (or at least where the error is). If it prints out all your default.vcl, you're good!