Some months ago, I was asked to review a paper for a computer science conference. I submitted my report about a week before the deadline, and I strongly recommended acceptance subject to a few minor corrections.
Just after the deadline, I was contacted by the programme committee member who originally asked me to review the paper. He sent me another reviewer's report (without a name or other identifying information) and asked for my opinion of it. The other reviewer had recommended rejection on the grounds that an example in the paper supposedly contradicted the claim of the main theorem. I explained that the other reviewer has misunderstood the example (in fairness, the authors of the paper could have stated it more carefully), and I see that the paper was ultimately accepted.
I have never before seen or been asked to comment on another reviewer's report, but my field is primarily mathematics, where there is often only a single reviewer for a paper, and I have only reviewed a few computer science papers.
So my question is this: in fields where it is usual to have multiple reviewers, is it common to be shown and asked to comment on other reviewers' reports?
A secondary question would be: if it is not uncommon, does it mainly occur when reviewers take opposing positions on a paper, like in the situation I describe?