The *First* Mrs. Buchman
In S6E15, "The Second Mrs. Buchman," Jamie is upset that Paul never told her he had been married once before, even though he assures her it was "only a paper marriage" so that a nice Russian woman could get a green card.
I see a couple of significant problems with the way this is presented:
1. Paul claims that the whole thing only lasted a couple of days. Of course he may be saying that just to calm Jamie down, but it can't be true. In the late 70s (when Paul's first marriage apparently occurred), I was casually acquainted with a couple consisting of an American and an immigrant, and they told me they were having frequent, required meetings with Immigration officials over a significant time span (a year or two, I think). They were taken into separate rooms and asked questions about their home life, in order to verify that they both said the same thing.
2. I suspect the other thing may be a deliberate set-up for a later episode, but Paul says nothing about ever getting a divorce from Svetlana (and Jamie doesn't ask). Legally speaking, there is no such thing as "only a paper marriage." In order to marry Jamie legally, Paul would have needed a divorce -- and the same for Svetlana, who appears to be re-married as well.