This is best explained in a video but I'll summarize here:
If you create more than one Qt window with a unified title and toolbar, and then send one window into fullscreen mode and back (or even just set unified title and toolbar to false on one of them), the unified title and toolbar will disappear from the other windows when it most certainly should not be affected in any way.
See video showing the situation here: http://download.petroules.com/videos/toolbar-bug.swf (code seen in the video is attached to this post)
If you have any questions about the situation post a comment and I'll provide whatever information I can.
I tested this using Qt 4.7.2, 4.7.3, 4.7.4 and 4.8.0 (master in Git) and all versions are affected. I also tried with 4.7.3/Carbon and the issue seems to only affect Cocoa builds of Qt.
I expect the issue lies in src/gui/widgets/qmainwindowlayout_mac.mm but my knowledge of Cocoa is very limited so nothing in particular stood out. There's also a comment near the beginning of QMainWindowLayout::insertIntoMacToolbar(QToolBar*, QToolBar*) that I found interesting:
"Switching unified toolbar off after this stage is not supported"... I don't know if that is related.