I'll tell you what the problem is...
you have more than one textbox on your page and you are calling validateField onBlur of both/all of them.

When you enter text in the box 1 and then go to box 2 box one is being validated. setting the focus to box 1 causes box 2 to be validated which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated,which then sets focus to box 2 causing box 1 to be validated, which then sets focus to box 1 causing box2 to to be validated

...and so on!