Reference on how often "To the best of our knowledge" assertions turn out to be false Quite often authors make some claims with some "to the best of our knowledge" disclaimer. 

Example:


  "To the best of our knowledge this is the first time this approach has been used to address this problem".


Is there any research/study/survey that tried to look at how often such claims are false? (e.g. some counterexample can be found)

I am mostly interested in the field of machine learning / data mining in English-speaking venues, however any reference on occurrence of such false assertions would be of value.
To the best of my knowledge, it is unlikely anyone has done the gruelling work of trawling through the literature, looking for such statements, and then doing the detailed work of finding out if the claim was true.
On the other hand, if I state such, you'd have to prove I did have prior knowledge to refute my statement. In itself, that could come close to an accusation of plagiarism in the context you cite. Or at least of sloppy checking of previous work.