I've been coming home from work and painting, scraping, touching up, whatever for a few hours each day this week... can you tell?
(1) These newfangled painting trays coated in Teflon (let it dry; peal it out) are really cool and work realy well...
(2) ... unless it is primer that you let dry. Then it's a toss-up as to whether it would have been better to scrape it out or wash it.
(3) Just because a gallon of paint didn't un-mix significantly in the 2 months between when you bought it and when you first used it doesn't mean that it won't un-mix visibly in the 2 days between when you painted with it and when you used it for touch-up.
(4) Physics be d*mned. Just because you painted with color 1 first, and color 2 later, on a different day, doesn't mean you won't find a place where color 1 is splotched atop color 2.
(5) If your choice is pouring too much paint into your "cup" or too little, choose too much. Really. Yeah, you can always add more, but if there's too little, it'll dry up badly, more quickly than you expect it to, and if there's only a few drops too little, and you're anything like me, you WON'T add more, you'll just work at oozing as much as you can out of the brush. And when bits and pieces of the brush are coated with already=-ried paint, things get weird.