Apollo, Soyuz and Orion have used, are using or will use launch escape towers. If it is not needed, it must separate during flight. What if it does not separate? I suppose that the spacecraft would drop its upper stage and land. But these spacecrafts use parachutes to land. Isn't the launch escape tower in their way? And is it safe to land with the added mass of the escape launch tower?
That'd no doubt interfere with the reentry, and the deployments of the parachutes. I suspect they'd call an abort slightly after the failure and maybe actually use the tower to escape, but I'm guessing there. Perhaps it'd be risky to activate it if didn't get jettisoned correctly, who knows how it is remaining attached..
Details on redundancy jettison operations from apollo sim: http://nassp.sourceforge.net/wiki/Launch_Escape_System
And links at the bottom of that page to official docs.