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One of my professors says this often happens with circular island chains; populations from any two adjacent islands can interbreed, but not those from islands farther apart. I don't have a source. Presumably this doesn't require an expanding geographic barrier.
I'm thinking of a situation where there are subspecies A through (say) H; A can interbreed with B, B with C, etc., and H with A, but no non-adjacent subspecies can produce fertile offspring.