There is a popular misconception that watercolour paintings cannot be altered once they are finished. In fact there are a number of techniques which can be used to rectify any mistakes, one of which is lifting paint off with a damp brush or sponge.
Some friends pointed out a compositional mistake in one of my recent paintings, and once I had seen it I wasn't happy to leave things that way. The offending watercolour is shown below and has a line of fence posts, which look too small, and which lead the eye out of the painting.
By lifting off the paint with water and a brush I was able to remove the posts. I then painted over the area and used a clean, damp brush to soften the edges. I also lifted off a bit of paint to make the remaining posts a little taller. The result, I think, is a much better painting and shows that it is always worth trying to rescue any failures.