Smear occurs when a very bright portion of an image causes an entire column of pixels to overload and bloom to white
CCD sensors experience light smears. Smear is also one of the “dead giveaways” that your production was shot on video rather than on film; film doesn’t “smear” like this. Controlled lighting can eliminate all traces of smear, but in uncontrolled circumstances it’s going to happen. CMOS sensors function differently and are immune to smear.