Bash – did the “d” break when I screwed up a bind

Your gnome-terminal (actually the underlying vte-0.34) emits the wrong sequence for ctrl+Alt+space. The bug (https://bugzilla.gnome.org/show_bug.cgi?id=710349) was fixed in vte-0.36.

If you're not afraid of hacking a little bit and you're able to safely revert things in case of trouble, you can try to install vte-0.36 on your Ubuntu 14.04. You'll get many other fixes and improments along with this one. You might find a PPA or a package in Gnome3 staging, or compile it for yourself. Upgrading to this version of vte doesn't require touching any other software components. A complete restart of gnome-terminal is required (close all the windows).

The terminal emulator translates events like “the Tab key was pressed” into sequences of characters that the application running in the terminal (bash, in your case) reads. See How do keyboard input and text output work? for a more detailed presentation of this topic.

For historical reasons, a few of keys send a character that's the same as pressing Ctrl with some other character: Tab = Ctrl+I, Return = Ctrl+M, Esc = Ctrl+[. This is because historical physical terminals did this, so applications that run in terminals expect it, so terminals do it.

Both Guake and Gnome-terminal use the VTE library, which does not allow the mapping from key chords to character sequences to be configured. You have the same problem as bash - wrong key sequence bindings with control+alt+space

Xterm has fully configurable key bindings. You can make the Tab key send a tab character (that's the default), or make it send the string hello, or whatever you choose. Xterm is configured via X resources. For example, to make Tab send the escape sequence \e[t when pressed and \e]t when released, put this in your ~/.Xresources:

XTerm.vt100.translations: #override \
  <Key>Tab: string("\033[t") \n\
  <KeyRelease>Tab: string("\033]t") \n\

Or maybe you would leave Tab sending the tab character and make Ctrl+I send something else:

XTerm.vt100.translations: #override \
  Ctrl~Meta~Shift<Key>I: string("\033[a5i") \n\
  Ctrl~Meta Shift<Key>I: string("\033[a6i") \n\

You can then bind \e[a5i to whatever you want in bash and other terminal applications with configurable keybindings.

Note that by convention, multi-character escape sequences start with the escape character (often represented as \e or \033 or \x1b in programming languages and configuration files); some applications may have trouble with escape sequences starting with other characters, and of course you can't have a character that's both an escape sequence and a key of its own, unless you're willing to accept a timeout (that's how it works in applications such as vi where Esc on its own is bound to some functionality). If you define your own key sequences, take care not to clash with the ones sent by function and cursor keys, which are more or less de facto standardized.