You may have figured out by now that these mistakes are in no particular order.  Each of them can be equally disastrous. 

This particular one shares one unique characteristic in that it is a mistake that can and often is also made by people who have paid good money for a well-constructed estate plan!  Ugh!

It is best explained by this example: 

A married couple had two ne'er-do-well children, a son and a daughter.  The daughter was apparently worse (as far as the couple knew) because the couple created wills specifically disinheriting the daughter. (Yes, you can do that).  They kept their wills at home in a desk drawer.  Some years later, the son, for reasons unknown, murdered both of his parents in cold blood, was arrested and put in jail pending trial.  The daughter broke into the house (or maybe she had a key), found the wills and "disappeared" them.  Now that the parents were apparently intestate (died without wills), the daughter stood to inherit half of the parents' estates along with her brother.  But wait!  Almost every state has a "slayer's statute" that provides that a murderer cannot profit from his crime by inheriting from the victim, as this state did.  Accordingly, and assuming the son would ultimately be convicted, the daughter stood to inherit 100% of the estates.

The parents' mistake was failing to keep their wills in a safe place where it couldn't be stolen, and also safe from fire.  Some people leave their wills in their attorney's safe, but that is a throwback to the days when safes were not affordable for the average family, and attorneys liked keeping the wills so they could be guaranteed the probate work.  Now small fireproof safes are quite affordable.  Just remember to give the key or combination to someone you trust.  Hint:  It should not be someone you are disinheriting!

Another option is a safety deposit box at a bank, but I am not a big fan of this method for several reasons, one of which is it is really hard to obtain access just at the time when you need it the most.

If you are not worried about theft but mostly just fire and flood, there are very affordable fireproof and waterproof bags available online.  Just remember to make sure your loved ones know where to find it when they need it.

To the attorneys who are reading this:  Yes, I know there are legal remedies for this situation, but as we all know it is truly an uphill battle and very expensive.  First, you have to find a definitive copy of the will that was "disappeared".  That in and of itself can be a huge challenge.  Then you have to overcome the presumption that a will that cannot be found was intentionally destroyed with the intention of revoking it.  In other words, you would have to prove beyond a shadow of a doubt that the daughter found the wills and destroyed them.  Circumstantial evidence may not be enough.  In the example mentioned above, many remote family members that stood to inherit if the will could be proven just couldn't afford to undertake the litigation that was going to be necessary.