The other day I used my friends orange cab so I switched my head to 16 ohms. I forgot to put it back to 8 when I switched to my 2x12 (8 ohms) and played a 2 hour practice that way. The amp sounds fine and hasnt exploded or caught fire... Did I dodge a bullet or is there some permanent damage?
Dodged a bullet.
Well made amps with good transformers can usually run that way without serious damage. Mesa transformers are actually spec'd to be able to do that with no harm. Their transformers are good enough to run with power tubes missing o.O