Once again I have come across in my editing an example of a misused group of words that are almost homonyms rather than definitive homonyms; they deserve a post, though, as they can be easily confused.
Ensure: to guarantee; to make certain
Insure: to provide or obtain insurance on something
Assure: to give confidence to; to convince
Allow Me to Illustrate
If your friend is heartbroken because her douchey boyfriend dumped her (again), you assure her that things will get better.
But, if you are secretly in love with your friend, you will do everything you can to ensure that her happiness is restored, even if it means giving her ex a beat down.
If you go through with the beat down, you had better hope he doesn't try to sue, as there is no way to insure yourself against that lawsuit!