In this screenshot, which of the two tabs is the current document?
This problem is worst when there are exactly two documents open. With three, then there's an odd-one-out that is brighter. But with only two, there's no indication whether bright or dark means 'current'. In fact, I'd say brightening the current document's tab is misleading, since if you continue the tab metaphor, the tab with a background colour closest to the canvas (i.e. darker) creates more of a tab-like affordance.
This does actually cause me a problem. After a month of using the application quite regularly, I still find myself clicking the wrong tab.
