Should I consider watercolor wash outlines mistakes?
You get these outlines when the pigment collects on the edges of the water you have laid down. This happens when you use a lot of water, causing the pigment to float around in the "puddle" and gravitate towards the edges as it dries (if you're interested in the physics, there's even a name for it: the coffee ring effect!).

To avoid these outlines, you need to use less water when you lay down the color. You should have enough water to wet the entire area, but not so much that it puddles on the paper. You may find that you need to dilute your pigment more - before the wash - to achieve the same color. (Note that diluting pigment is not the same as using lots of water to apply it!)

This will involve some amount of experimenting to get a "feel" for how much water is necessary. Also, consider using a smaller brush since you mention yours sometimes holds too much water. I would recommend trying different ratios on a scrap piece of paper to see what works best with your pigments.