Your camera sensor may develop problem pixels.
Problem pixels are caused by defective photosites on the sensor.
Photosites are the structures that collect the photons.
Some of the photosites on your sensor will develop excessive charge leakage.
When the current of the leakage is added to the current produced by the photons, you see a problem pixel.
There are three varieties of problem pixels.
A stuck pixel is one that's always too bright, regardless of the shutter speed.
A hot pixel is one that's too bright when using long shutter speeds.
Don't worry about hot pixels that appear at long shutter speeds and ISO settings you don't normally use.
For example, if you're not doing astrophotography, don't worry about a hot pixel that becomes evident only at long shutter speeds.
Hot pixels can be any color.
Green hot pixels maybe more prevalent, because there are more green photosites on a sensor than red or blue.
You can have hot pixels that are not red, green, or blue, if there are two defective photosites side-by-side with different colors.
The two colors mix to form a single color, such as cyan (blue + green), magenta (red + green), or yellow (red + blue).
A dead pixel is off color.
For there to be a problem pixel that's black, the red, green, and blue photosites for a pixel would have to be defective.
That's rare, so most dead pixels are off color.
Don't worry about problem pixels that you can't see—without using hot pixel mapping software—or long shutter speeds and high ISO settings that you rarely use.
If you have distracting problem pixels, they can be mapped out by the camera manufacturer.
Mapping out means that the photosites next to the problem photosite are used to interpolate (estimate) the value of the problem photosite.
Camera manufacturers map out all sensors before they leave the factory, as no sensor has 100% operating photosites.
A few cameras can be mapped out by the user.
If there are many adjoining defective photosites, the sensor may have to be replaced.
If you use long exposures, your camera may have a noise reduction feature just for these long shutter speeds.
The Adobe Camera Raw converter removes most problem pixels.