Banding Noise
Horizontal or vertical stripes appearing in heavily lifted shadows, caused by variations between readout channels.
In depth
Because a sensor is read out through many parallel amplifiers, tiny differences in their offsets produce stripes rather than random grain. The eye detects structured noise far more readily than random noise, so banding becomes objectionable at levels where equivalent random noise would be invisible. It is a common limit on how far shadows can be pushed.
Examples in practice
Lifting an underexposed interior by four stops reveals faint horizontal stripes across the darkest wall.