Craftsy blog posted this post on the 15 most common knitting problems. I concur with them! Most of them are frustrating!
1. Running out of yarn just before finishing a project! To get around this, I’ve always tried to over estimated yarn (my husband hates that I have a box of scrap yarn!). Sometimes, depending on the project, I buy a full skein of excess yarn. That way I know I won’t run out. It has saved my bacon 3 times I know of in the past.