As far as hearing goes, there are non-linearities in the human cochlea that can produce perceived difference tones at certain volume levels and frequencies.

Quantization noise in small enough digital signals might also create enough non-linear mixing to show up in FFT results.  Noise shaping in the quantization could help reduce this possibility.

Any other non-linearities in the system are also potential difference/sum intermodulation/beat tone sources.

Two generated tones creates a side tone - how to avoid the side tone
I have some danish users that uses special chars like æøå and they can't log in to my wordpress blog. I tried on a fresh install on 4.2.2 and on 4.1.5 and it did not work, for example Andrew Schønnemann with a simple password and it did not work, got message ERROR: Invalid username. Lost your password?. But with user Andrew Schonnemann works fine, what could be the issue?
