Repairs that disappear under raking light.

On a flat or lightly textured wall, yes, with enough feathering and a primed patch. Heavy or distinctive textures are harder, and a knockdown or orange peel match is done by testing on scrap first. Raking light from a nearby window is the honest test, and it is what the work gets checked against.

Because joint compound absorbs paint differently than the surrounding painted surface. Without primer over the patch the finish coat sinks in at that spot and reads as a dull halo, even when the surface is perfectly flat. Priming the patch solves it.

Movement, usually seasonal, and usually at a stress point like the corner of a door or window opening. Filling the crack again will not hold. The fix is to address the joint properly, often with tape and a wider feather, and where the movement is structural it is worth finding out why before repairing the surface.

The work itself is quick; the drying is not. Each coat of compound needs to dry before the next goes on, which typically means multiple visits across a few days for anything larger than a small patch. Rushing a coat is what produces a repair that cracks or shows later.

Sometimes, if it was caught quickly and the drywall has not lost integrity or grown mold. Drywall that has been wet long enough to sag, crumble or stain has to come out, along with any wet insulation behind it. The source of the water gets fixed first, or the repair fails again.