This is really a Doh! moment since the behavior is obvious in retrospect.
I created a Spark program that used updateStateByKey and thus required a checkpoint directory.
After running the program, there were a few errors to be corrected. Subsequent runs still exhibited the same behavior, leading to some concern that sbt and Eclipse were conflicting, etc.
Of course, the sunsequent runs read the contents of the checkpoint directory. That includes serialization representation of the impacted classes!
Deleting the checkpoint directory resolves the problem.