The title says it all. Suppose you are refereeing a paper where the author A makes strong statements about other papers by a different author B, like: the proof of Theorem 1 in paper [B] is wrong and we provide an alternate proof.
One possibility would be to diligently read the other paper, compare the results, and decide. This doubles (or triples) the refereeing effort, and puts the referee in a more difficult position, opposing both author B and the authority of the other journal and its referees, in case he decides to support A's claim. I am not sure this is the proper course of action, especially when the disputed result is not of such importance to deserve an exceptional attention.
Another possibility would be to ask the managing editor to involve (also) the other author B in the refereeing process, asking his opinion. The potential dangers in following this way are clear I think.
What do you do in such situations?