Over on the BAUT forum, someone pointed out this image
http://www.bautforum.com/attachments/quest...813_0918_c3.jpg
I looked through the SoHO images to be sure it wasn't a hoax, and this image did appear on the official sight with no obvious artifact like this in either the image before or after it.
So it is obviously not a giant comet with a coma twice the diameter of the Sun which only appears for a few minutes... it is probably an artifact inside or right next to SoHO. Does anyone have a good guess as to what it is?