Key repeat while shell dialog is presented
You can unexpectedly cause key-repeat with GTK applications (presumably others, but untested) if you are typing while a Shell dialog is presented.
To reproduce, I do something like this in gnome-terminal.
git pull
# start typing next command anyway while i wait for it to complete
then the git pull
prompts for an SSH key-phrase.
While that dialog is visible, the underlying terminal continues repeating the last key.