So you are positive you are creating just a plane? and not a cube by accident? are you maybe touching the 'g' key which is 'repeat last command' ?? cos this is really bugging the hell out of me...I cannot replicate your problem. Can you post a quick video showing us what you do please...or at least some stepped screenshots?
cheers bullet
bullet1968
"A Darkness at Sethanon", a book I aspire to model some of the charcters and scenes