funny.. my dealerships.. all 3 of them, say it doesn't void the warranty. That's only going to be the case if everything works.
You can destroy the head unit under the following conditions: some mentioned above
wrong impedance speakers
short on startup.
short duration operation
speaker failure during operation
the following will destroy (over time) high wattage handling speakers hooked up to the HU:
playing the radio loud and distorted! (the amp will clip sending square waves to the speakers which will burn the voice coils precisely at the amplitude the HU caps out at. (and leading up to there)
poor youtube quality audio files. (same as above) highly compressed music crams as many frequencies it can into a narrow band. This effectively has the same result as above when amplified... and worsens if both conditions (this one and the above) are met.