Actually, I'm pretty sure you've found one of two bugs in Indirect Light that are plaguing me. This problem is random, and it makes hideous dancing square artifacts in animations. I'm using the Mac version of C3 and I've found that increasing Photon Count above 2000 seems to make things worse. Are you on Mac or PC?

See if the problem goes away if you re-render with Indirect Light turned off.

BTW, the other bug I've found: glowing objects do not illuminate their surroundings. If I suspend a glowing sphere above a white plane, there is no illumination. Strangely, the photon map looks good. Even more strange, if the glowing sphere is placed near the intersetion of two perpendicular white planes (or any other objcects that would reflect light at each other), the planes illuminate each other with the invisible reflected light!

This image is looking down about 60 degrees over an upright plane with a 2" glowing sphere on the other side suspended a couple of inches above the floor plane. It illustrates a really bad instance of the random square problem. Also, all visible illumination is IL reflected from the upright onto the floor - take away the upright and nothing but the glowing sphere is visible.

If anyone else has time to try this, please let me know so I can say if this is a Mac-only problem when I send a bug report.