If the paper begins to lift or pill after repeated colouring what technical mistake has occurred?

Correct answer: Overbrushing a wet area

The governing concept is respect for the paper surface and drying time. Repeatedly brushing a wet or partly wet layer agitates the paper fibres, weakens the surface, and can produce lifting or pilling. The correct remedy is to stop, let the area dry, and apply fewer controlled strokes in the next layer. Cleaning the brush, allowing paint to dry, and selecting suitable paper are helpful practices, not mistakes. Therefore, option A is correct.