Thanks for the note, I'll try to explain what's happening here...
When LastPass fills in the forum login form it inserts the username into the `username` field as below:
At the bottom of the page, you'll see there's a newsletter sign up form - this uses a service called phpList (it's what's used for the Pi Weekly emails). This form contains an `email` input.
Since login forms often contain either a `username` or an `email` input (and the two can be used somewhat interchangeably), LastPass is automatically filling in this `email` field with your username at the same time as filling in the login form, the assumption being that this may be part of the login form.
When the login form is submitted, pre-filled by LastPass, LastPass detects that another form has information in it filled in by LastPass (the newsletter form) and it then presents you with the following warning:
Somewhat ironic since it was of course LastPass which filled in this form (unnecessarily) in the first place!
The phpList form never gets submitted upon log in, and it only ever contains your username (not your password), so it is seemingly safe to ignore. However it may be preferable to see if LastPass can be configured to not blindly fill in any matching inputs in the page that aren't part of the login form itself. Unfortunately we can't rename the phpList form's `email` field to anything else, because it's a required field!
Hope that helps clarify.