Drawing stuff to the screen. It takes some time, you figure it out, it's great, and then some time later you realise you want something like a different origin point or to flip the canvas and suddenly everything is horrible.
My drawing stuff is pretty set up to just draw in world space, where 0,0 is the bottom left and Y is up. And that's great for things like tile-based levels which tend to go up. It really sucks when you want to write a GUI and want to start at the top and work your way down.
But then you flip your camera, flip your text, and suddenly baselines are off and your newlines go the wrong way.
And each draw function invocation is an unreadable line of 100 characters long because it's filled with a bunch of parameters.
I was aiming for Saturday with this project but that mayyyyy not quite be the case. Who would have thought a game dev would miss a deadline :)
Here's a great archive of a bunch of different old GUIs for inspiration: http://toastytech.com/guis/index.html.
Update: A caret appeared just before the day ended!