Here's one that's been bugging me for a while.
When I paint a room, I cut in the edges and corners with a brush. The edges always look streaky and discolored compared to the rest of the wall.
Using name brand paint and a narrow high-dollar brush. I'm not saying which paint because someone will inevitably say, "Well there's your problem right there... Brand XXX paint is no good. Get brand YYY."
You've got this beautiful roller texture up to about 1" from the edge, then spooge. Even cutting in with 5-6 coats vs. 1-2 with the roller on the rest of the wall.