Sometimes even the best writers seem to take the easy way out, presenting something implausible or character-inconsistent to the reader. For example, I was reading an Ellis Peters book (and I esteem Ellis Peters-- I read her books over and over), and there was a scene where Cadfael (the monk-detective) double-crosses a knight (who is amused and will become a friend). The "MacGuffin," as Hitchcock would call it, is some treasure of silver and gold plate that Barringar (the knight) knows Cadfael has. Cadfael wants to give this to a young couple who are fleeing England's civil war. But Cadfael, when caught, sits on a burlap sack full of clanky things as if he's trying to hide the treasure. And Barringar falls for it and seizes the sack.
So far so good-- the implausible part comes after. The young couple get away with their goods (and the treasure, of course), and Cadfael allows the knight to escort him back to Shrewsbury (keeping him occupied so that he doesn't go after the youngsters). The knight talks merrily of how, when he gets back, he'll open the bag and have the treasure to fund his marriage. But... he never opens the bag. And of course, we know and Cadfael knows... the treasure is on the way to France with the young couple.
Why doesn't Barringar open the bag? Because the author needs him to be occupied and not chasing the real fortune and the young lovers. And that's not a good enough reason. It's not plausible that he would ride along with the bag tied to his saddle and not at any moment stop and look. Humans aren't built that way ("look, shiny!"), not to mention he knows Cadfael is a sneaky old bugger.
But Peters just doesn't make that plausible, perhaps because she didn't realize that it would jolt with us as "unlikely." So often we don't notice when something is implausible or inconsistent in our own story, though it would really stick out when we're reading someone else's. And it's often necessary to have this thing happen, for example, Barringar -can't- open the bag too early or he will discover it doesn't contain the treasure-- 1) so that the young lovers can safely escape, which he wouldn't allow if he knew the truth, and 2) he has to show that he has underestimated Cadfael's brilliance, and 3) he has to show that he is capable of laughing at himself later.
But you see, all this can be accomplished AND also be plausible. So if you were writing this, how would you make it plausible that Barringar doesn't open that bag?
I'd have some reason he couldn't open it here, like it's chained closed and he needs equipment to open it, and that's back at the castle. So that bag can't just be burlap (easily cut with a knife), but maybe a couple layers of leather, or chainmail (oh, btw, this is medieval, so no Samsonite :). And of course, it doesn't have to be a bag. Anything that can be tied to a horse (a small trunk?) could work.
So what's a simple, believable reason why Barringar has to bring the bag intact back to the castle before he opens it?