I should add the two most common causes of alternator failure I have found
are jump starting another car and over-tightening of the belt.

The belt does the obvious, it overloads the front bearing and either the
bearing will go out and seize or it will overheat the alternator so the
electronics inside fail.

Jump starting is not so obvious.  What I have come across is that there are
huge voltage and current spikes that occur just as the other car fires.
What happens is initially running/charging puts a heavier load on the
alternator.  Then when cranking the engine, the load increases tremendously.
At the time it fires and you release the key, the load requirements drop
completely and suddenly all the extra current causes a huge voltage spike,
plus the second car's charging system has also just added to the voltage.
Yes, I know that voltage on parallel circuits will remain the same across,
however, there is much more current.  This increase in current causes a much
larger voltage spike across internal components and blows out the regulator
or one of the diode packs.  The diode packs are rated by current, so if the
current is too large, out goes the diodes.

Sometimes they short, so then instead of getting rectified DC, you continue
to get AC, which means during a half cycle you are discharging the battery
on one phase.  this usually means that you end up with a dead battery in a
month.

If the phase is blown out - no current in one phase - then you end up with a
weak charging system that after a hard start (long cranking in winter, left
lights on, etc.), the system may not recharge the battery completely in a
short trip and then you have the "dead battery" and can't start, but in
longer trips, you have no problem.