I noticed weird things in opera 9.02 on windows xp with the built-in javascript validation for forms: it doesn't work at all.

Note that I have attached an extra class of 'required' to every field in my form that is required.

I seem to remember though that I once noticed that it did work but instead of saying

Please fill out "Paswoord", it is required.

it said:

Please fill out "this", it is required.

If you need more details, I'd be happy to check them out...