Making it too good. Sounds silly, but as a self-funded indie developer there was no one to tell us to stop, or not to add that feature. You get caught in a loop of “if I add this feature it will be more awesome, and more awesome games sell”.
I learned this the hard way. My business partner at the time summed it up as, "Every feature is a support call."
More stuff, even if you think it good, is more stuff that can go wrong, or introduce a subtle bug, or just not work correctly for someone, somewhere,