The pedigrees and group sheets on ancestry.com and the other online sites are very convenient, but when you rely on them, you are relying often on other people's work that we really don't know the integrity of (especially the LDS site.)

It has always been the case in genealogical research that bad information is passed around, but with the coming of the internet this process happens a lot faster. It really is heart breaking when you see an entire wing of your genealogical edifice come tumbling down because it was based on a foundation of incorrect assertions regarding more recent (or even more ancient) events. After it happens a few times, it become merely irritating, and one kicks oneself for taking shortcuts and not assessing information more stringently.

I have a cousin who says he never accepts anything as a fact until he can hold the original document that supports it in his hands. I think that's a very good rule of thumb.