Like others before me, I have found the Devil, and he is in the details. Little details, such as a typo, a wrong date, a word spelled wrong, whether someone was born "circa" a date or "before" a date. Just find a detail and you will find the Devil.
There are 418 Devils in place names in my FTM 2012, many of which are where I put the descriptive place, i.e. the manors owned, in the place name for birth. Since the majority of the records are of nobles holding many manors, the descriptive line makes the place name field go crazy. Other place name "errors" are where towns have changed their names; where the first name in the place field is not a town but a manor or a castle; when there is a slight change of spelling over the centuries that the place name authority doesn't recognize or when the data is simply wrong, as in where there is one city in two counties for a birth record.
I am fixing the place name errors a few dozen at a time, and putting the new records in the correct fields. It is just one part of the learning curve, about real estate in the Middle Ages, about the geography of England and France, about how Family Tree Maker works with online records, and most of all - about paying attention to the details, and keeping the little Devils out!
I have corrected most of the marriage date errors, and put contracts made for marriage while the subjects were still children in a different kind of record. That still leaves actual marriages made between people under sixteen, or in one case I found, under five years old. My program thinks it is a mistake, (I'm inclined to agree with it) but that doesn't change the fact. So I have resorted to adding wording indicating that it is a valid child marriage which was not repudiated (as was allowed by Church law) when the parties were of the age of consent.
Another kind of detail I have found is where the date for the birth is actually the date for the Christening, (which may or may not be the actual birth date), which is normally so noted, but which I have to remember to enter into the descriptive field.
I could list other little 'detail Devils' but you get the picture.
Then there is the detail of Copyright license and citing the author/publisher of a text or picture properly. I have almost totally given up using the convenient "add this to my tree" button in Ancestry - unless the article in question clearly cites the source and license (which I have usually found to be not the case). Sometimes I can find the original of the (unsourced) item on Wikipedia or elsewhere, and I can use excerpts giving credit where it is due. More details, more Devils.
Excuse me, it is time for me to go kick another dozen little Devils out of my work.