Unwanted reflection of screen color onto foreground elements — ruins keying, requires repositioning lights or resetting screen distance.
You know the problem: The bluescreen wall is wonderfully saturated, the lighting is perfect, but then in playback or later during keying, you see that your main character has a blue halo around them — especially at the edges, in the hair, on the shoulders. This is blue spill, and it occurs because the light from the background wall bounces directly back onto your subject, illuminating it with the screen color. This makes keying a nightmare because the keyer can no longer cleanly separate foreground and background.
The physical reality on set: Bluescreen material reflects light — partly diffusely, partly directly. If you position your talent too close to the wall or have cut the lighting unfavorably, the wall acts like a large, blue reflector. The light bounces back and tints hair, earlobes, and the shadowed sides of the face and body. The finer the material (fabric instead of plastic), the more diffuse the reflection — but also more problematic because it's everywhere. The classic situation: You think the lighting is perfect because the wall is evenly lit, but only realize during the keying process that your talent is completely tinted blue.
Prevention and remedies: First — and this is the best solution — distance. At least 1.5 to 2 meters between talent and the wall. Second: Backlight and side lights must be precise. A hard backlight (Fresnel) optically separates the figure from the wall, breaking up the halo. Third: A green screen background instead of blue can help if your talent has very light or reddish tones — but this is not a universal solution. Fourth: In keying itself, you can work with channel-based keying (split channels) to isolate and subtract blue spill components. Some keyers also use despill tools that specifically correct blue edges.
The most important thing: You don't solve blue spill solely through better keying. The right way is on set — through distance, intelligent lighting, and the use of contrast lights. What you don't get right there will haunt you in post-production.