Re: Multiclip

First nothing on the mirror will ever show up on your pictures or video, the mirror is out of the way when taking any image.

Is this issue happening on more than one lens?

If there is a sensor problem you need to have canon look at it. Sensors should be be cleaned by professionals, there are kits out there is you are professional. The other thing maybe a few bad pixels.

Is auto sensor cleaning enabled? if not enable it and do a couple of manual cleaning. If that does help post a sample picture of the problem.

There is also the Dust Delete function under menus you should also try that.