On OS X, open a Swing application with menus containing mnemonic (SwingSet2 for instance). For instance, File menu has a visible mnemonic set on F.

Now, with Aqua, Java (Metal), and Motif LaF, one can open the menu pressing Ctrl+Alt+F. So it is reported in sun/lwawt/macosx/LWCToolkit.java. getFocusAcceleratorKeyMask(). By default, SunToolkit reports Alt for this mask.

And so, inconsistently, it works with Nimbus. To open File menu, you have to press Alt+F even on OS X. It's clearly a bug.

Separately, a question: could we believe the output of getFocusAcceleratorKeyMask? Is there a public way to find it at any moment other than looking in the current LaF settings?