Why is my customer's email appearing on a different Contact's timeline?

Sometimes a customer will email Contact A but notice the email ended up on Contact B's timeline

This is an item I've encountered a number of times, and if it's not cookies on a shared machine it's usually an email that was forwarded from the intended recipient and clicked by the forwarded recipient.

In this particular case it's a matter of the tracking pixel from an existing email being copied into another email by the user copying EVERYTHING in the email.

As Philip Porwisch mentioned in Slack when this conversation came up:

Usually what happens is the following:
  1. The customer writes a great email to one of their contacts.
  2. They then decide they want to send this email along to another contact, so they
  1. go into the old email and copy the whole thing
  2. forward the email and swap the previous recipient's details for the new recipients details
  1. In the case of 2a, the tracking pixel gets also copied. I believe this doesn't result in associations to the wrong contact, but every open will be counted for all emails
  2. In the case of 2b, however, the original email's id (from the email header) get's associated with the new email's id. I.e. all following emails are tied together as a thread and we bring these associations into the system as well.
The result: all does emails appear as one thread on the timeline of each contact, that has received minimum one of them. So I would check in with the customer if the above is the case and if yes, recommend them the use of templates instead