Wikipedia quite rightly wants citations for stated facts and this practice goes a long way to prevent inaccuracies.  But I recall one instance where it actually caused the establishment of a factual error.

Some city in Germany had a new mayor elected and someone duly updated the Wikipedia article on the city to reflect this.  Another person made a correction--the new mayor's name had been misspelled.  The Wikipedia editors rejected the correction, citing a published article in a major newspaper that had the name spelled as it was in the Wikipedia article.  After some back-and-forth the spelling correction was eventually corrected.  It seems that the newspaper in question had gone to Wikipedia to find out the new mayor's name and so the original Wikipedia misspelling had gotten published in print.

-Paul W.