Biggest one, for me, was trying to save Segritt during the Battle of Haven in my first playthrough. Mind you, I figured out quickly enough how to get inside the burning building. So I find him laying there on the floor, smoke and fire all around us. But then I couldn't figure out why he kept laying there even though I was clicking on him over and over and over again ... hadn't yet figured out, you needed to right-click and hold on a character in order to revive them.
Then I couldn't figure out how to get the door opened, because the debris was in the way. Managed that eventually, but Segritt died during that playthrough. Poor Segritt.
Then there's trying to get up to the top of any particular incline. I'm edging my way up there, carefully enough. I'm almost there ... almost. Then I suddenly slide down on my backside, again. And I'm cursing up a storm and my kids are looking across the room at me. And THEN I look up at the top of the hill -- trying to find some better pathway up there, mind you -- and I see Solas just standing there at the dang top!! And it always seems to be Solas, by the way. Oh my friggin' tricky elf god, sometimes I hate you!