Denormal numbers
Floats so close to zero that the CPU drops into a slow path — the classic reason a silent reverb eats your processor.
IEEE 754 lets values below the smallest normal exponent taper off toward zero with shrinking precision instead of snapping to zero abruptly. It's mathematically elegant and practically expensive: on many x86 chips they're handled in microcode, and a single operation can take a hundred times longer than usual.
Audio code falls into this trap more than almost anything else. A feedback filter or a decaying reverb tail keeps halving its value until it lands in denormal territory — precisely when everything has gone quiet. The result is CPU spikes and xruns the moment the music stops. The fixes are the FTZ and DAZ flags in the SSE control register, which round such values to zero, or the old trick of adding an inaudible DC offset or a whisper of noise.