I don't know why it happens, but if I press the Left Ctrl the output is backtick (`). When I press the right Ctrl the output is like Ctrl+Z. My keyboard layout is USA.
Well, I don't know what the problem is, but you might be able to force your way out of it using Xmodmap.
It requires the following steps:
1) Find out what keycode is written by your left Ctrl key: start a terminal and run 'xev'. Then place your mouse over the Event Tester and press the Ctrl key. You will get a read-out like
This was for my Left Alt key. Locate the keycode -- in this case '64'.
2) Enforce a different behavior: go to your home directory and edit '.bashrc' -- if you use Nautilus and not a terminal to find it, then it's hidden: Press Ctrl+H to show hidden files.
Add a line to re-code the key behavior:
Of course, the 64 should be replaced for the keycode you obtained in step 1. Now save the file and close it.
3) Activate the behavior: Run a terminal. Every time the machine has been booted, this behavior has to be activated -- this is done by opening a terminal. You can close it right away again. Otherwise, you have to make the change in a different file, [see here].1
I hope this solves your problem!