Yes, its true. thats why in game jams you have to do game that takes you half the lengh of the jam to make so that you can test it, iterate and bugfix. I also make that mistake and scoped a game that would take me 3 days. The problem is that you aren't going to make the game straigh away going straight to your idea like a behemoth, you will find flaws in design, and you will go back and forth with some things. Thats why i didn't completed my mission for my gam