When I see people taking off a cover of a phone, they often release one edge and then really pull the cover to make the other edge come off. Doing that with the Desire cover WILL warp it. The material is too soft for such a stress to be put on it so I'm not surprised some say the cover won't fit back on again.
Be smart: release one corner (with the indent), then use a knife along the edge to pop the other corner. Then it won't get warped.