I warn clients about putting factual statements in their will or trust that later might change or be proved wrong.  Even if the statement is true when the will is written, if the circumstances change, it can create grounds to contest the will.  Fro example, if your will says that you are giving someone an unequal share because they are getting your life insurance, there may be grounds to challenge the will if your life insurance lapses or if you decide to change your life insurance beneficiary.  If you merely stated that you had chosen not to give that person an equal share, there would be less to contest.

The example cited in several law school classes involved a man who thought his wife to be unfaithful.  By proving that fact wrong, she was able to successfully contest the will. 

Forbes blog recently posted on How to Prevent A Will Contest.   However, in my opinion some will contests occur even with the best advice and preparation.  A will contest that was bogus but still resulted in undue time, expense and hassle is described by attorney Robert Fleming in this blog post.  Robert also has a good post on the rarity of successful will contests.    However, If you carefully read the descriptions posted by a law firm on this site, the prospects of a will contest can be depressing.  Many of these cases described resulted in "confidential settlements" or "a significant portion of the estate" going to the party who seemed to be in the right.  What I read from that is that the person who should not have received anything still got something, making the will contest worth their while. 

Attorneys Murphy and Goodson have a fairly comprehensive study on will contests in their state of Arizona.     I share their view that we should work to prevent these contests where we can.   One way to do that is to carefully consider whether there are any statements in the will that can later be proven false, even if true at the time the will was made.