What is the hardest thing you have encountered so far, why was it hard and did you sort it?

For me it had to be the inventory system, just the amount of if statements that were used killed me slowly, and the variables kept on changing. I have only done 6 items for the inventory and it is already 400 lines of code and that's just telling the game what everything is I'm around 70% complete, a few more 3d models so touching up and a few more scripts along with fx is all I have to do then i'm done!