Comment 6 for bug 382473

It's a setting in the keyboard layout. The issue is that the right alt is being mapped for 3rd level characters despite the fact that there are no 3rd level keys in the US. I still think it's a bug, but here's the fix for KDE:
goto System Settings > Regional & Lang > Keyboard Layout
Under 'Layouts' Enable 'Keyboard Layouts'
goto 'Advanced' tab
open 'Key to choose 3rd level'
Uncheck everything except 'Right Alt key never chooses third level'

There's some other interesting settings in there too - useful for customization nuts.
I imagine the setting is similar in Gnome.

Here's the command code too: setxkbmap -option lv3:ralt_alt