It's a real problem, because Wikipedia's trustworthiness depends on its verifiability policy. Everything in Wikipedia is supposed to be traceable to a reliable source. Unfortunately, Wikipedia itself has become so trustworthy that supposedly trustworthy sources are becoming too uncritical about trusting Wikipedia.
Back circa 2004-2005 a respected editor added a statement to an article saying that Rutgers had been originally been invited to join the Ivy League but had declined. This interesting, plausible, and credible statement was in the article for a while, but was eventually challenged.
The editor originally had trouble providing a good source, but eventually came up with a newspaper article in a New Jersey newspaper, one that would usually be considered a reliable source. Other editors were inclined to accept, this, until one of them realized it was a fairly recent article, contacted the reporter, and asked for the reporter's source.
The reporter replied that he had read it in Wikipedia and used it (without attribution).
Now, it's not clear whether or not the statement is true. The last I knew, the editor said he had gotten it from an old issue of the "Targum," the Rutgers University newspaper, which would probably have qualified as a reliable source, but since he was unable to provide volume, issue, date, or page numbers, the statement was not verifiable at that time and was removed.
But it is an clear example of circular reference--an unverifiable statement almost being kept in Wikipedia, based on support from a "reliable" source that had gotten it from Wikipedia.