Like you, I have no intention of testing this in the field.
Real Life Experiences
I haven't seen where someone just reverse wired one battery, but I have seen where both batteries have been installed in reverse.
One time, I went on-call to a place where the NAC Power Supply was sending trouble. There I found that a maintenance person had replaced the batteries earlier in the day, but put them in backwards. In that case, because of the way that NAC Power Supply was designed, installing the same batteries correctly fixed the problem. The NAC Power Supply had circuitry that prevented damage when the batteries were installed backward.
With another NAC Power Supply brand though, when the batteries were installed backward, the soldered-in fuse on the NAC Power Supply burned out. I suppose one could say that the charger circuit was "Fried" because the whole Fire Alarm Control Panel had to be replaced because the soldered-in fuse was burned out.
I won't test these theories out in the field because testing is too expensive; replacing Fire Alarm Control Panels and NAC Power Supplies is not my idea of inexpensive testing.
I have theories, though, for two conditions: one theory is with no charger but the batteries are supplying power, and the other theory is when the batteries are being charged.
No Charger but the Batteries are Supplying Power
When a forward 12-volt battery and a reversed 12-volt battery are installed in series, the sum of the voltages equals zero. Because the voltage is zero, no current will be drawn. When the batteries are not being charged, nothing happens.
By Douglas Krantz Check It Out
Batteries Being Charged
Sealed Lead Acid batteries have very little internal resistance, so a forward 12-volt battery and a reversed 12-volt battery connected in series constitute a dead short. This, though, is when they are first connected to a charger.
If there's an internal fuse on the charger, the fuse will probably burn out. If there isn't a fuse, probably the charger has some sort of current limiting. This part is mostly a design issue for the charger circuit, and the design will be different for each model of Fire Alarm Control Panel or NAC Power Supply.
If there isn't a fuse, however, what happens to the batteries is interesting. Keep in mind that the batteries are wired in series so whatever current goes through one battery goes through the other. One battery will receive this as a forward charging current and be forward charged, and the other battery will receive this as a reverse charging current and be reverse charged.
The forward battery is going to be charged at the full charging current. When it is fully charged, it will continue to receive this full charging current and become over-charged. Over-charging a battery that way will kill the battery in a short time.
The reverse battery will be reverse charged. Reverse charging a battery never works very well. The reverse charged battery will not go to full voltage in reverse so the charger will continue to try charging at a full charge current. Reverse-charging a battery will kill it much quicker than over-charging a forward charged battery. Once it even starts to be reversed charged, consider the battery to be dead.
Batteries will only become warm when there is current running through them. Watts law is involved with heating the battery. If the internal fuse on the charger circuit is not blown, and the charger is trying to charge batteries where one battery is reversed, the batteries will become warm because of the full charging current. The batteries can be considered bad if they are warmer than the box that they're in.
Quick Test - Always Feel-the-Batteries
I have found that under normal use, even when the fire horns have been sounding for extended time on battery backup, the batteries will not feel warm because the time that current is drawn is too short to heat up the mass of the battery.
Every time you service or test a fire alarm system, feel the batteries. When the batteries are warmer than the box that they're in, both batteries are receiving full charging current and both batteries are bad. In that case, the batteries are being over-charged and need to be replaced.
Perform the battery tests like you normally do, but this quick Feel-the-Battery test can be done first. If the batteries have failed the quick test, you don't have to set up to perform the normal battery test. Just replace the warm batteries.