When sublime is in the background with multiple buffers open, clicking on the application in the area of the tab bar will activate that underlying tab while also bringing the application into the foreground. This is incorrect behavior according to HUD guidelines: clicking on the application area should just bring it into the foreground and should not activate any of the controls (in this case, the editor tabs). Note that clicking in the editor buffer area does not behave as an activation (e.g. you cannot change the active cursor position by clicking while the application is in the background).
Basically the only interaction which is supposed to ignore clickthrough is scrolling, which Sublime does correctly.
This is of course a very minor issue.