Here's the one that has bugged me the most lately, because it has such impact on the story itself. I've posted it elsewhere, but ...
If the charity dance at which Jack met Randall happened in 1978, according to the banner over the stage, and presumably Jack and Randall started seeing each other not too long after that, and if Ennis met Cassie sometime just before that, then ...
... by the time Jack and Ennis each discussed their relationships with the "good looking little gal in Riverton" and the "ranch foreman's wife," those relationships had been going on something like FOUR F'IN YEARS. (The screenplay in "Story to Screen" says three, but that's unmistakably flawed, based on the chronology of events following that lakeside scene.)
Anyway, it's illogical that they would have gone four years (or even three) without mentioning these other involvements -- AND YET, when the subject happens to come up in conversation, they nonchalantly spill as if it's no big deal. If they were keeping these involvements secret for years, fine. Or if they had only been going on a short while and so they don't mind casually discussing them, fine. But not both!