I'm pretty sure it's just a case of the autocomplete feature of their browser putting the user/pass that's set to login to the site when the page loads and they are submitting the forum with those fields populated with that data...

Which would not be a threads error, but a problem with the browser and the user just not validating the information is correct when submitting the form.

I believe I read something somewhere that they where planning to have an autocomplete=off for fields on some browsers, but I don't know if they made any headway

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