It's not a bug. Too bad you didn't tell what OS you run or I could tell you exactly (or a link) on how to fix it without tapping NUMLOCK.
I recently got a problem using my laptop. It's a 3000-N-100 Lenovo model and every time I try to write letter "u" it comes number "4". When I write letter "i" it comes number "5", when I write "o" it comes "6", when I write "j" it comes "1", when I write "k" it comes "2" and when I write "l" it comes "3".
I don't really get what I did wrong, but now I can't write and I don't know how to repare it.
Do you know what to do?