I'm a bit late, but I'd say the Duck Change (every time I try it the two cards go in completely different directions, often without even turning upside-down) and the pinky count, but not because I don't understand the mechanics, in fact I have a pretty good one, but because for some weird reason I can't naturally use it while performing a trick.
I just realized today the one I've been working on for years, the one trick I want to do (as part of a couple effects) that I won't do for people because I am sooo inconsistent, the Toss Change.
It's tricky, and I always seem to flash past the pinky.
Every time I think "I've got the positioning right" I'll do it one more time and flash. I want to be good at it so bad, it would go really well with a couple of my DLs... alas.
It's interesting to read how different people struggle with different sleights. I've always had no trouble at all with basic card palming and can even use the hand concealing the card pretty, umm, handily... but try as I might I just cannot achieve a consistent, quick and secure backpalm card vanish. I just seem unable to consistently hit that sweet spot where nothing is showing of the corners and the card is secure. And I've tried for years!
Echoing someone else here... sometimes the stuff most regard as basic trips us up the most. My overhand shuffle control is still pretty sketchy, even after many years.
I just have to drop in again to say that following Justin's advice on another thread re the Duck Change, I have now all but mastered it, and I am so damned pleased with myself! If anyone else keeps having the issue of the cards spinning out sideways, take Justin's advice of placing your second finger a little way in from the top corner of the cards. So many of the tutorials I've seen suggest that you should have it right on the corner, barely holding the cards securely. This is what causes them to spin out to the left (assuming you're holding them in your right hand). You need to have a very light grip, but it does need to be secure. Justin's tip of practicing with one card is also gold. Just work on getting that one card to do the "flip" first, before you even think about adding a second.
If I could sum up what cracked it for me it would be that second finger position, and being as light as you can with the pressure applied to the card(s). Don't over-bend them. Just enough to get them to pop out when you simultaneously release from thumb and fingertip with gentle pressure from the forefinger. Also, don't hold your right hand too far from your left. Use trial and error to get it as close as you can while still leaving room for the turnover. This helps stop the cards separating on landing. But seriously: practice with one card at first. You don't even need to try to catch it in the other hand, just do it against a desk top, close-up mat, couch you're sitting on... keep trying to gently ping that thing downwards until you get it. Like most subtle sleights this is one of those where you have to manage it successfully (or nearly) once in order to get that "ah-ha!" moment. Then you know roughly how it should feel and you can reproduce it more easily.
More progress: I've now nearly cracked controlling the cards into the left hand so that they stay together and don't fly out and separate. Here's how.
Angle the right hand slightly downwards before you release the cards. Aim them approximately (but not quite exactly) towards to heel of your left hand. This takes most of the momentum of the flight away, so they only move a little bit forward, and hopefully settle more or less square with the deck.