To give you an example, today whilst searching any new documents for my GGG Grandfather George Robert Miller's marriages I had forgot to untick the 'Family trees', being curious I had a look to see who had him/them in their tree. I spotted one that had Mary Ann (Deliser) Miller with a place of birth as Kent, England, as far as I am aware no researcher to date has found her documented arrival to Australia or where she was born, if you have please reveal yourself! I was excited, WOW, then I looked again, and again. This tree had her being born in 1822, convicted in 1815 at Middlesex and marrying George Miller in 1833 (unsourced), interesting. Then I spot she has 2 George Miller's attached as her husband and some children are doubled up. Even more interesting, Mary is living back in Kent in 1851 after her death in 1849. Seriously scary misinformation, so I clicked on the "Member Connect" button to see that there are 11 other family tree's this person has gathered information from or is connected to. Oh boy, what a mess.
George is already a difficult person to sort out, see a previous post laying out the children and the names used for their mothers http://www.dixie9.com/2009/05/george-robert-miller-debate.html.
My lesson to you, PLEASE stop and research the information before adding it to your tree, just because it's on Ancestry does not mean it is correct. You can't just enter your name and have your family history revealed to you like the ads indicate, it's not that simple! The tree's on ancestry are the work of individual people that have gathered information over the years, some correct, some incorrect, some speculative. Make notes and source the information for yourself to prove in your own mind that the information is correct. The thing is, I can also understand how it happens, the sheer excitement of connecting the dots and thinking wow! and having a whole family line unfold right in front of your eyes. Just slow down, keep your tree private until you have sourced your information ie gathered the documents to prove the dates, and then when it's ready share your tree with the correct information.
Below is the family group sheet of Mary Ann (Deliser) Miller as mentioned above so you can see for yourself, the identity of the family tree has been removed.