The following query returns an incorrect answer:
This should return the row 2, 1 but instead it returns null, 1. If you permute the order of the columns in the select statement or the order of the joins then it returns a valid answer (i.e. one without incorrect NULLs).
This turns out to be due to two longstanding bugs in GenerateUnsafeRowJoiner, which I'll describe in more detail in my PR.