The dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video. It consists in animated checkerboard patterns which appear along vertical color transitions. It results from intermodulation or crosstalk between chrominance and luminance components of the signal, which are imperfectly multiplexed in the frequency domain.
Dot crawl only appears when the luminance is transmitted with a high bandwidth, so that its spectrum reaches the band of frequencies used by the chrominance signal in the composite video signal. In other words, it appears when "too much detail" is transmitted.
Another, similar problem is the appearance of a colored noise in image areas with too much detail. Television studio guests usually have to avoid clothing where too fine patterns appear on clothes, eg. ties.
Dot crawl was first widely noticed with the Laserdiscs.
It can be removed using a comb filter in the receiver.