If I press "ALT+Space", and then "x", the current window is maximized. However, if I press "ALT+Space" again and then press "m" (which is the highlighted letter in the menu), it just selects the item, instead of unmaximizing the window.
I'm trying to reproduce here the bug, but I think it's not valid.
First, 'alt+space' and 'x', window maximized, then 'alt+space' and 'x' again, window unmaximized. The problem is that 'm' is the mnemonic for "Move". What's the distro and language you're using?
I'm also moving this bug to Xfwm as it seems more appropriate.
Ubuntu 14.04 and vanilla Xfce.
Ubuntu Studio 14.10. The interface is in Brazilian Portuguese, and these are the entries in the menu:
* Minimizar todas [a]s outras janelas
(the letters marked with  are the ones underlined)
It's seems to be a mnemonic conflict, as you can see 'm' is used in "Des[m]aximizar" and "[M]over"... luckily, I'm one of the translators for pt-BR, so tonight I'll fix this and update here.
Thanks for the heads up.
Translation updated on Transifex, eventually it'll show up in master.
But do note that currently if you press 'alt+space' and 'm', you have to press 'enter' to the action(Unmaximize) take effect, otherwise if you press 'm' again it'll cycle to Move.