Version:           1.12.0 (using KDE 4.3.0)
OS:                Linux
Installed from:    Fedora RPMs

Kmail launches external editor when control key is depressed.  I finished editing my message and saved and closed my editor.  The compose window now has my edits.  I want to send the message, so I attempt to hit Ctrl-Enter to send, but the control key alone launches another editor process.  I have to click the send button to avoid another editor process after exiting the editor.

kdepim-runtime-4.3.0-1.fc11.i586
kdepim-runtime-libs-4.3.0-1.fc11.i586
kdepim-4.3.0-4.fc11.i586
kdepimlibs-akonadi-4.3.0-2.fc11.i586
kdepimlibs-4.3.0-2.fc11.i586
kdepim-libs-4.3.0-4.fc11.i586

kmail-1.12.0

In earlier releases, the control key had no such effect -- it took another key depress to launch the editor.  I don't know exactly when this behaviour was introduced since another bug prevented me from using the external editor until quite recently.