Ensure means to make certain that something will occur or take place; insure strictly means to arrange indemnity for or against something or someone. The misuse of in- for en- can lead to confusing and sometimes humorous implications: I shall ensure your death means I will make certain that you die, but I shall insure your death means I’ll arrange monetary compensation in case you die—the minute difference between a capitalist and a villain. This mistake is fortunately rare, yet even The New Yorker, which takes pride in its unique prescriptive snobbery, has confused insure for ensure more than once.