A "broken" or "hot" pixel. Mileage about 10,000+, removed "run", vosstanovlena firmware (after a failed firmware or use Magic Lantern), illumination of a matrix of laser beams — "drawdown" of color, contrast one pixel at a time or all at the field, after hours functions — clean image sensor, shooting modes, broken shortcuts, non-working quick combination of buttons) and/or the recovered box (damaged blinds, mirrors), "broken" electronics, operation outside the permissible temperature (humidity, pressure), extreme shooting (shock, overload of more than 10g). All of this is that you when you buy will never be able to determine, of dismantling the apparatus or not by checking the brand service ON Canon or using proprietary software or tools on a portable laptop (Adobe CS, Adobe Bridge, DeadPixelTest). Here only the most General list. Full testing may require more than one hour, inspection camera, special. tools (mirror dentist, loupe, macro ring, white LED backlighting, etc.)
I can tell you cant have, or not be. I took the "new" in the well-known brand. I slipped Mark II with a broken pixel and the mileage 10000.
Tip — take a new in a sealed factory box and don't let anyone open it unless you know how to search for schools like the back-focus of the lens or light box.