of a flap failure, failure to detach the tie-downs before taking off, a piece of cloth blown onto the tail, the previous pilot forgetting to remove the dead horse from the back, you name it.
Getting silly now...
Meanwhile, in the real world, the pilot had no problem correctly diagnosing the problem and taking the appropriate action... Bit strange if someone flying a familiar airplane could not tell 2200 RPM from 2600 RPM from the sound of the engine. Bit stranger if engine RPM was 2600 RPM with the throttle pulled way back simultaneously with the ASI underreading (2 simultaneous independent failures). So there are indeed ways to crosscheck the indications.