I've heard of cases where a Sephardic family, for whom rice is totally kosher for Passover (assuming it's checked and contains no additives etc.), cooked rice in a pot. The next day, they invite their Ashkenazic neighbors over and cook completely kitniyot-free, but their Ashkenazic neighbors say it's a problem as even potatoes cooked in that "kitniyot" pot is a problem.
Am I getting this correct so far?
If so, is there any halachic basis to this, or is it "just one of those Passover customs"?
This should, in theory, be far better than a case of a non-kosher pot; by some quick back-of-envelope math, for any normal pot I can think of, the ratio of volume-of-the-pot-material-itself against volume-contained-within-the-pot should be far less than 50%.