Once in a while I’ve had the problem of being halfway through typing an email or an answer to one and lo and behold the email becomes “Sent”. I had disabled keyboard shortcuts in Gmail thinking this might be a contributor to the problem but in fact this is not the case. The “error” occurs when by design or accident the ‘Tab’ key is pressed; followed by the ‘space’ bar or the ‘Return’ key. What happens is that the Tab key changes the focus to the Send button, hence when you hit space or return the button is effectively ‘clicked’ and the email in progress gets sent. There is apparently a whole debate on this but Google, probably correctly IMHO, argues that they should not redefine the standard action of the Tab key on a web page which is indeed to switch between fields. Now that I am aware of this, I’ll be watch my typing, especially near the Tab key which on my French keyboard is right next to the ‘A’ key.
There is a Google labs widget “undo send” to allow you to unsend an email.