When you run out of space to add things that made me mad
Insertion of new tiles is the devil. I still don't get it. I have a go at it, fail, get more frustrated than I've ever been, put it away for a while, then repeat the process. (All the time renewing the "frustrated than I've ever been.")
Check out my sig, theres a link to my tutorial on adding tiles with a-map. If you need help just send a PM.
Right now, probably inserting graphics like a titlescreen or battle backgrounds and getting just a white screen when testing.
Probably, you did something wrong :P I could offer my help inserting a 256/240 titlescreen :) for you :P