The typical cause for this is that you (or someone else) has accidentally clicked a tracked link in an email that was sent to the lead. You may have clicked the link in your copy of an Outlook email that you sent them via Send and Track or they may have replied to your email and you clicked the link in their reply email or someone forwarded you the email originally sent to them and you clicked the link. In any case, that causes your cookie to point to their lead record. You can confirm that this is the case by going to a form that pre-populates the lead info and if you see the lead's data instead of yours, you know this is the problem. You can also have your co-workers do the same thing in case one of them clicked the link. If the form has the lead's info, you should delete the _mkto_trk cookie for your web domain and that will stop the erroneous activity from being logged into your lead's record.
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