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I'd say there's no need to change your password big ben. I too find that if I want to log in on the Home Page, the site won't recognise the password first time of trying. Second time is OK.
If I'm reading a topic and decide to log in from there and post on it, the login details go in first time.
It's just a small thing that happens and perhaps nothing to worry about.
I have that too, I wonder if Google saved an old one, I've tried putting the current one in but it still happens.
I try to reply and it says to log in then it does so with the saved password on the forum.