I wouldn't be surprised to see something like the OP describes happen
I don't think you thought about this carefully. Going from "(+0 -2)" to "(+0 -1)" would require votescast to be decremented. We never decrement it.
What does happen sometimes is that updates get lost. If both parts of the update got lost, then the reputation display simply wouldn't include that vote. It would not go "backward".
If only half of the update got lost (only votescast getting incremented or only reputation getting inc/decremented), then you'd see something like "(+0.5 -2.5)". People would remember seeing such a thing so I don't think that has ever happened (which makes sense, considering how the node cache works, such would be quite unlikely though perhaps not completely impossible).