We all use Ancestry.com and value its services in making images of original records widely available. But even a profitable publicly-traded corporation makes mistakes.
My friend and colleague Michael Hait, CG, has started the Ancestry Errors Wiki as a place where genealogists can share their reports of cases where Ancestry.com has made errors of imaging, organization, or programming in placing records on line -- such as placing part of a census in the wrong jurisdiction. Simple errors of indexing or transcribing individual names can be dealt with on Ancestry's site, but these deeper errors are more difficult to detect, publicize, and correct. (This wiki can help with the first two items, anyway!)
In January 2009 I discovered one such error in the 1865 Illinois state census, in which townships in Kane county were mislabeled and one entire page image omitted. I wrote it up for the Illinois state quarterly. That was two years ago and it has yet to be corrected. In this case there are workarounds and Family History Library microfilms for comparison; in other cases there may be alternative on-line providers with better quality control. But first researchers need to help one another by making the problems known when they do occur.
So far the wiki has notations of errors in records involving seven states: Connecticut, Illinois, Kansas, Kentucky, Maryland, Missouri, and Texas. May you find few errors -- but send them in promptly when you do!
(I don't pay much attention to anniversaries, but if you're counting, this is post no. 801.)