Builtin VNC stops working after waking headless Mac mini from sleep (black screen)
This was eligible for me: edit /Library/Preferences/SystemConfiguration/com.apple.Boot.plist like this:

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;Kernel Flags&lt;/key&gt;
    &lt;string&gt;darkwake=0&lt;/string&gt;
&lt;/dict&gt;
&lt;/plist&gt;


and restart your mac. Mountain Lion forces me to enter password twice but it wakes up and works.