I didn't start my legal career doing much estate planning; for the most part, I was a probate attorney, handling Wills that had "matured" into deceased estates. I think that's a helpful experience, as you quickly figure out from a practical perspective the difference between good provisions and provisions that fail.
Let me give an example from a Will (not drafted by me, luckily) that recently came across my desk. The Will gives most property in equal shares to the decedent's three living children, which is fine. But it also makes a gift of certain jewelry to "the first of my granddaughters to marry." Setting aside the potential inequality here -- why favor the first granddaughter only? why favor only granddaughters? -- there's a problem: none of the decedent's granddaughters have married. So what now? At the time the Will was drafted, this provision failed (it didn't work), and it still fails today.
As a probate attorney, the problem is clear. I have to file with the probate court a document listing the decedent beneficiaries (legatees). Who in the world do I list from the above provision -- all of the granddaughters? someone who can hold the jewelry in trust until a granddaughter marries? somebody else?
If the draftsperson had spent five minutes thinking about the practical ramifications of this language, he or she could have easily fixed it.