It could be several things. The turbo will be over-speeding, the engine will have to work much harder to exhale through the turbo and to compress 30 psi of pressure, the turbo will be putting out higher temperature air than it should, etc.

All of these are bad, but they will not cause engine failure in minutes or even hours.

While this is all occurring the car will still drive fairly normally, so the average driver would not know anything was wrong and would likely just drive it until something eventually failed. That is why I believe that they installed the over-boost protection system. Low power is a sure way to get a driver to take the car to a mechanic, and limiting the fuel will also limit the amount of over-boost the engine can make since it can no longer run at full load.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine