A downmix is a sum, and a sum can clip.

Lo = L + clev × C + slev × Ls. With the standard −3 dB coefficients that is a gain of 1 + 0.7079 + 0.7079 = 2.416 — +7.66 dB — applied to content that was already near the top. So three channels sitting at −6 dBFS, every meter comfortably under zero and nothing wrong anywhere, fold to +1.66 dBFS.

The engine is free and needs no account. Nothing in the multichannel mix shows it: the clip happens in a decoder downstream, on a device nobody in the room was listening to, and the mix it came from passes every check you can run on it. And it is not one fold — six of the nine standard coefficient sets clip that bed, and the only survivors are the ones that throw the surrounds away. The page also works out what the centre does when it is copied into both outputs rather than routed, why only −3 dB preserves its power, and how the same surround channel can be the reason a fold clips or be entirely inaudible in it — 4.65 dB added or 10.69 dB gone, decided by phase alone, with nothing on a meter to tell them apart. Every lane ships a worked example that costs nothing. Writing the report needs credits.