Think about the reasonableness of any information you find in a compiled database. Are people having children before they are born? Are people getting married after they died? Are there individuals who get married before their parents are born? If you have never seen entries such as these, you've not looked at very many online trees.
Personally I use online compilations as CLUES, sometimes very weak clues, to give me ideas. Never incorporate such information into your database and never spend an inordinate amount of time trying to prove it. But once in a while, compiled information in the "online trees" is correct.
But make certain when you compile your own information that you are not violating any laws of space, time, biology, or physics.