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.