allears4u, I've made the mistakes you mention. My worst chronic mistake has been speaker placement (ironically, a zero-dollar aspect of this hobby).
Placement oversight/neglect seems common. Based on the "my system" pictures posted throughout the web, it's apparent that many people place their speakers to agree visually or fit architecturally in a room, rather than to optimize the sound.
Another category of mistake I'll mention is when to listen critically (for acoustics) and when to listen to music (to enjoy). I think I ping back and forth in a sub-optimal way.