Canon Rebel T2i Quick Start Manual page 35

Shutter Speed Distortions
In the following two images you will see an example of a type of error that
can occur when shooting video of an object in fast motion at high shutter
speeds.
Notice how in the second image, the blade of the fan is distorted.
This distortion is caused by the electronic shutter of the camera.
The electronic image scanning is slower than the movement of the fan
blades. We don't notice this at slower shutter speeds because the slower
shutter speed registers only a blurred image of the fan.
This type of error occurs on all DSLR still cameras shooting video. Even the
most expensive models costing thousands of dollars.