Colour spread too far while softening an edge in watercolour. What was the likely mistake?

Correct answer: The damp brush contained too much water

The governing concept is water control in a watercolour edge. To soften an edge, the brush should be damp rather than loaded with free water. If it is too wet, it deposits extra moisture that carries pigment outward, so the colour spreads beyond the intended boundary. A completely dry layer normally will not spread by itself, and insufficient pigment makes a mark weak rather than causing excessive flow. Blotting the brush before use prevents this mistake.