Andy, they just take all the sources and copy them over to the wrong person now. Having sources is no assurance that the material is correct. The problem is most people don't actually read fair amounts of what they have in their trees. I ran into this with a document that was important for my family. The document was located in one place but clearly and repeatedly gave another location as the place of death. I had to repeatedly point out that within the document was the place of death. Several people copied the document and did not bother to actually read where the place of death was.
One of the trees out there copied an item that I had added. It was a photo -owned- by my family of Ulyssees S. Grant and if you read the description it states this, but it was copied over as a photo of my ancestor. I am not descended from Ulyssees S. Grant. The comments didn't help. He is still an active member as he went on to add the name of a living family member and indicate that he is deceased. Two comments from two separate people have not yet cleared that up either. I believe that tree currently shows the most sources for that family member in the hint listing.
How do you enforce critical reading skills or the ability to evaluate material? This isn't done by voting or copying.