I'd say that numerous mysterious aspects of the timeline provide the biggest "hunh" moments for me.
For example, the amount of time that Ennis dates Cassie and that Jack presumably sees Randall works out to be about 4 or 5 years if you look at various clues in the movie (such as the banner in the Childress dinner-dance scene). Therefore, these sideline relationships are fairly significant... if only for their duration. The fact that Ennis seems to only be mentioning Cassie for the first time during the final camping trip, when Jack asks him if he'd ever get married again... always seems a bit odd to me.
Yep, sure enough... great TOTW!