A terrain glitch is a bug made by terrain generator when generating this part of celestial body's surface. It may create freakish terrains, sometimes affecting the gameplay. It might be considered an easter egg, but those are probably not intentional.
This is the most usual type of terrain glitch. It occurs as a very strange terrain on the poles of celestial bodies, like Moho or Eve. It's most likely a mistake happening when terrain generator attaches height values to a sphere. This can also happen (less often) at other significant locations, for example at the coordinate origin (zero degrees north, zero degrees east). As well as the images below, this glitch is known to occur at the north poles of the Mun, Duna and Ike, but will happen on almost every planet with land at the pole.
Terrain at Eve's south pole.
As is shown in its name, this is a glitch where terrain cracks. It provides an "entrance" to the inside of a celestial body, but unfortunately, when flying something(like a poor Kerbal) into the crack, it will be destroyed immediately since it's beneath the surface. This often happens around the poles, due to odd terrain, see Polar Glitch above, but is not exclusive to the poles. As well as the images below, broken terrain is known to occur at the North Pole of Minmus.
Map View discrepancies
Since the texture used for celestial bodies in map view (or when in space high above a body) is a different file to the texture used when the terrain starts loading in, there can be discrepancies between these views. A good example is Eve: if zoomed in all the way in the map view (or Tracking Station) the ocean will change from being very shiny to a dull purple, and the land brightens slightly too.
Below are some images of a dark patch on Ike, which only appears when approached. One could speculate that the magic boulder crashed into Ike, leaving behind an oddly behaved crater which delights in confusing the player.
A close up view of the dark patch on Ike.
There is a known glitch with terrain scatter which can sometimes leave a rock apparently floating in mid-air.