This is observed on Nimbus L&F, on all OS, with the current 6u4b02 pit build.
Minimized JInternalFrame does not have an opaque background. Apart from the buttons(Maximize.Minimize,Restore) the rest of the minimized JIntFrame's shape is transparent and blends with the background.
Steps to reproduce:
1)Run SwingSet2 with the current 6u4b02 pit build and in the JInternalFrame demo, minimize the various JInternalFrames.
This is more clearly visible when you place the minimized JIntFrame over another Unminimized internal frame.