Weird Visual Effect

I was with the Oklahoma City Astronomy Club holding a public star party for some local college students the other night, and I had brought along my Discovery 8" Dob. I had plopped in my Nagler 13mm T6 and was letting people look through it when one of the other club members asked if he could plop it in his scope and try it out. He has a 4-month old Orion XT10 Dob, so I assume the mirror and mirror coatings were still in great condition. Anyway, he put the Nag in and asked if I wanted to see it in his scope. I said "sure" and proceded to look. At first, an odd effect scared the shiz-nit out of me because I was afraid something had happened to my Nag, but I quickly discovered that it must be something in his scope. The "odd effect" looked kind of like orange flames just along the inside edge of the FOV; it was along about 30 degrees of the circumference of the FOV - if that makes sense - when vieweing Jupiter. I did not get to look through it at anything other than Jupiter, so I don't know if the effect would still be present on something like M31. So my question is, what was this effect? To me it looked kind of like the pictures of older mirrors that are losing their coatings toward the edges, but his new mirror should have been in excellent condition. Thanks!