Good day.
I've faced some key combination issues.
1. I have Alt+Return for maximize/normalize window.
2. Log into system, try to use it on several windows:
2.1 On xfce4-terminal: Return passes to terminal, no maximizing occur.
2.2 On Thunar window: Instead of maximizing I got directory properties window.
3. Then go to settings -> Window Manager -> Keyboard.
3.1 I got Alt+Return as maximize window.
3.2 Reset this entry, set Alt+Return again.
4. Then go and try maximize windows again:
4.1 Everything works.
4.2 After system restart combination is broken again.

$ xfce4-about --version
xfce4-about 4.11.1 (Xfce 4.10)

$ uname -a             
Linux aperture 3.13.0-29-generic #53-Ubuntu SMP Wed Jun 4 21:00:20 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 14.04 LTS
Release:	14.04
Codename:	trusty