I made a custom template with a fixed header image, and underneath it I load the content that is add in the editor, starting with a big image.
If I send out the preview through "send preview" it all looks fine, but when I send out the email to a test group I found there was a gap between the top image (the one I set fixed in the template) and the one I added in the editor.
I had a look with firebug, and I saw a 1x1 image was added to track who opens the email. Removing that image, or setting it to display none fixed it.
So, any reason that tracker image wasn't set to display:none to begin with? I would also like to think that the preview that is send out by "send preview" should have 100% identical code to the actual e-mail that is send out. Which is not the case, otherwise I would have noticed the broken design before. You can see the difference in the screenshots that I took in Gmail.
Based on the preview I would almost have send it out to 4000+ subscribers, who would have gotten an email with a broken design.
There is also a feature that I think would be nice to have, and that is to be able to set a custom thank you page for new subscribers and unscribers. Now it just goes to blank page with "You are successfully unsubscribed!" I think it would be nice to be able to send them to domain.com/unsubscribe/randomcode and write a custom text.