Multi-field update op may fail because one field updates a subdocument or subarray field and the sub document/array doesn't exist (because it was deleted by a later op). In that case, the entire update failed to apply. Here's a concrete example.

Before initial sync, the doc is {y: [0]}
--------- start clone --------- 
x = 1
x = 2, y.0 = 2 (multi-field update)
       y = 3
--------- end clone -----------
The expected state is {x: 2, y: 3}