To the best of my knowledge, there are no known instance of Wikileaks publishing fraudulent or altered information.
What they hold back, what they publish and when they publish it can potentially be abused - for instance to present or enhance one narrative, and obscure another.
And Wikileaks itself could be used as a tool by other actors ( e.g. for political gain) via strategic leaks or somesuch…
The only newsworthy item I have seen was this: http://mobile.reuters.com/article/idUSKBN12Y2WY
Of course nothing has come of this speculation, so it may end up being nothing.
My last $.02, given the sensitive nature of some of the information that Wikileaks releases, there is probably no avenue for corroborating the materials other than, maybe, how the source received said materials.
Not ruling it out, but if they get caught one time it calls into question everything they've ever released.