I've made a plugin for this: https://github.com/eproxus/focus_last_tab
Duplicate of http://sublimetext.userecho.com/topic/30102-ctrl9-should-select-last-tab/ Please vote there instead.
This. So much this.
Looked this up a bit, and it seems there's no way to set environment variables on the user level any more in OS X Mountain Lion. See this for more info: http://cumptrnrd.wordpress.com/2012/09/18/setting-environment-variables-in-os-x-10-8-mountainlion/
Also happens for Cmd+P and Cmd+Shift+P.
This happens on OSX as well.
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