What will happen if i plug a cabinet into the wrong ohm setting?

[mbengs1]

say i set the amp to 4 ohms and plug it into the 16 ohm of the cabinet? will the amp break or the cabinet? 

[Mr.Grumpy]

For tube amps, It can break the amp. An impedance mis-match (different ohms on the amp and cab) can damage your power tubes and/or your speaker transformer.  It should be avoided if at all possible.  What's bad is the amp will "work" more or less normally, so you might think "Oh, this is OK!" but you are really stressing your power tubes and/or speaker transformer and they could fail suddenly. (and expensively)

I think having too small of a speaker load is worse than having too much, because it can cause the power to tubes to arc (sparks) inside of them. Sort of like mashing a gas pedal down while a car is neutral and letting the motor wind up past it's redline. Having too much load (lower cab impedance than amp impedance) is like trying to tow a heavy load with an undersize car, it will heat up the speaker transformer and could potentially cause it to fail. 

Solid state amps generally don't care about speaker impedance as long as it's above a specified minimum, usually 4 ohms. 

[DeepEnd]

You could damage the amp but you're unlikely to damage the speaker unless the amp puts out more Wattage than it's designed for. If the amp still works you probably didn't hurt anything so consider this a cheap lesson in paying attention.

[Grant Harding]

For the 4 ohm output setting I suggest using a 4 ohm cabinet. Why go 16 ohms? Maybe try 4 x 16 ohm cabs hooked up in parallel?

[DeepEnd]

The short version is, if your amp has a 16 Ohm tap use that for a 16 Ohm cab. If it doesn't, either find a cab with an impedance rating that matches your amp or use a different amp that matches your cab.