This effect is a very common one and is caused by dead or damaged pixels.
Pixels may be "stuck on" and a single pixel producing a bright one colour dot or a small group of pixels
In images they are often termed "hot pixels. Most sensors have some. They are a common consequence of aging. Some sensors will have them from new, some won't.
LCD screens also have them. Software is available to map them and to adjust the image to greatly remove their effect. As a simplistic example, if you had a single "hot pixel" if you averaged the light values in the adjacent pixels and replaced the "stuck" value wit the average value it would be exceeding unlikely to be noticed in the very large majority of images. The algorithms used for correcting them are liable to be more complex than this but this explains the concept well enough.
Some cameras have a hot pixel correction function - a long exposure shot is made in 'total darkness' (eg - with lens cap on) and the camera then adjusts for anything it finds in the dark frame.
Hot pixels will usually be most obvious in areas where they stand out against the background, and in longer exposure images.
"Stuck" pixels are most obvious if they are stuck on - but pixels that are always off will also be encountered. Here's an image showing an always off greem pixel.
