This could completely depend on the car in question. Older cars may have a cable driven speedometer employing, well a cable and a cup and permanent magnet drive for the needle. Many newer vehicles employ a speed sensor that feeds voltage to an electro magnet driven setup and then there are digital display speedos that use chips and voodoo. In the latter two a voltage leak across the cluster circuit board can cause some weirdness as well as the vehicle speed sensor. Any other gauges acting wonky? Someone with basic knowledge of electricity and the volyage values for the sensor can run this down for you in minutes.

More than likely a bad instrument cluster. However you can get a cheap scan tool from harbor freight and monitor the speed sensor signal to make sure its operating correctly

Bad speedometer

If could be if the car had an electronic speedometer but since you didn't mention what type of car you have ....

No, it sounds like its the IPC. Output sensor has no control of where the needle stays, the cluster does that. Means you need a new cluster