I can tell you how.
A shitty Belkin KVM in certain configurations can allow this to happen.
There's a bug which keeps generating chr(32) characters when you activate the keyboard shortcut (scroll lock twice), and try to switch to another machine.
It will keep pumping out those spaces on whatever fields was selected at the time, so if you take your time before you switch back, you are going to be in for a lot of fun.
Haven't read the link yet, but wanted to share this sooner than later.