The cross-version update fuzzer can fail because it assumes upsert is deterministic, but it is not. This occurred in a nightly Evergreen run when an upsert used the $inc operation, which succeeds on numeric values but fails on objects, arrays, and strings with error code 14. The problem arises because any upsert with multiple matching documents will update a random one of them. (This is not intentionally randomized, but MQL does not provide any guarantee as to which matching doc will be updated.) Thus a doc that started with a numeric value in, e.g. "obj.num" may or may not still have a numeric value there after an upsert has updated a random document's "obj.num" field. Subsequent upserts therefore cannot know whether the operation will be supported or not on whichever document they (possibly also nondeterministically) choose to update.