Screen burn-in can happen on some types of screen and can cause permanent damage when a still image is displayed for a long period of time.
The display can exhibit areas of discolouration or darkening.
After many many hours of testing the app over the last two years one of my devices (a Samsung Galaxy S3) does now show some burn-in, which can be seen as a slight yellowing of the display where the speed and other information is displayed in landscape mode. Another older device (an HTC Wildfire) has no burn-in at all. The Samsung has an AMOLED screen whereas the HTC has an IPS screen.
Because of this I thought it worth making people aware of the potential for damage when using any app that displays a still image for a long period of time.
Please check your device documentation for more information on whether your device may be susceptible.