Something this happens but sometimes it does not and I don't know when.
Steps I used when I met this bug most recently:
(1) I input something in the search box. ("special:Userlogin\")
(2) Press Enter.
(3) Suddenly I realize I typed something wrong (one extra "\") so I press Escape to stop page loading. The content in browser window is not changed now.
(4) The text in search box is stripped to "special:Us"
(5) If I press any more keys, it replaces the last character. I press "l" and it turns into special:Ul
(6) If I press backspace, I cannot delete characters before ":".