You click on a link in an email and the fields on a form are pre-filled with a different person's information.
When you forward an email that contains tracking tokens, the links in those emails are dynamically generated for the original sender. If you forward that email, then the links will still point back to the original person who received the email. This will cause form pre-fill and all activities to be tracked under the original person who received the email.
To prevent this in the future, send an original email that has not been sent to a lead already.