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This is a follow-up to the previous retreat in circles thread. The conclusion there seems to be that a unit can retreat "back and forth" between two hexes if that option meets the retreat priority requirements. This situation doesn't come up often (thankfully), but it does happen at times when a unit is retreating along a major river - to avoid an EZOC, the unit can move back to a vacant hex it had just moved from. In the past, we have not allowed this "bounce back and forth" - it seemed like a "gamey" move. I just wanted to confirm that the move is allowed under the most recent rules version.
If the retreat meets the priority criteria as outlined in the rules, then yes, it is possible that you can get a yo-yo effect. This will only occur when (as you note) the retreating unit is running up against a major river. It's worth noting that you can't "voluntarily" avoid the EZOC, so if the unit retreats up against the river and then "bounces" it will be forced to retreat back into an EZOC and undergo a Hazard check.