Yes, if you check the device datasheet, the device will begin to break down or avalanche at 20V working voltage, it's supposed to be capable of flowing 110A repetitively once avalanche breakdown voltage is exceeded. This diode is on the power feed of the computer to protect it from voltage spikes and the intention is once avalanche occurs the 5A fuse limits current to just 5A.
The problem is, the diode once breakdown voltage is exceeded, in many cases is damaged and doesn't recover despite current through the diode is limited to just 5A by the sacrificial fuse. The diode is supposed to be capable of shunting 100A repeatedly but it's behind a whimpy 5A fuse thus if the diode worked as intended, no harm would occur besides just blowing the replaceable fuse. Once the fuse is blown and current stops, the diode should return to it's un-shunted state but the quality of this component unfortunately, or perhaps over time exposure to spikes, weakens it enough to make it unreliable.
If system voltage rises enough to cause the diode to avalanche, there's a good chance of it failing. It's not a good idead to test this unless you're willing to replace the MPEM or perhaps repair it somehow. I think there's a sticky floating around somewhere on an MPEM dissection for this reason.
So to be safe, never connect a power source that will exceed 12.5 volts. Yes, the regulator charging circuit will raise battery voltage beyond 12.5V, I measured mine at 13.8V while the engine was running. 13.8V is enough to fully replenish the charge (of an healthy battery) normally consumed by starting the engine in a reasonable period of time.
Some quality multi-stage battery chargers can take the battery as high as 17 volts briefly while charging, this voltage can trigger the avalanche to occur. This is why it's wise to disconnect the battery while a maintenance charger is attached, or while "BOOST JUMPING" the battery.
So there's no guarantee yours will fail if say a 17 volt source is connected, or even a 20V source perhaps, it shouldn't result in a shorted diode if the diode is working correctly, worst that SHOULD happen is the sacrificial 5A current limiting fuse blows. Attaching any power source greater than 12.5V is just a bad idea unfortunately b/c of the high failure rate. This isn't a design fault, it's a component failure.
Thus when I connect/disconnect or charge my battery, I turn the battery switch off first. Whenever my boat is stored unattended between uses, I also turn the battery switch off.
FWIW, it's not all that uncommon for a vehicle control computer to become damage as a result of some sort of battery event.
The transient supressor is triggered once the system voltage exceeds the reverse breakdown voltage of the diode and once this occurs it begins to avalanche, shunting current to ground as long as the voltage remains above the reverse breakdown voltage. This keeps the overvoltage from travling into the MPEM logic circuit, a type of electronic crowbar. As a result of the 5A fuse, the current through the suppressor diode is limited to 5A, well within it's capability it seems. Perhaps installing a smaller fuse might even keep it from failing, I'm not sure?
If the current being shunted by the diode once it begins to avalanche exceeds the capacity of the fuse (This will occur if the diode fails at any time or the transient is persistent) the fuse will blow and current will stop, the power source for the MPEM logic circuitry will be interrupted.
So if this happens to your MPEM, you might be able to repair it by going inside and replacing the diode, or even just cutting it out of the circuit (not easy or recommended).
https://en.wikipedia.org/wiki/Transient_voltage_suppressor
The ONSEMI datasheet provides more info for this surge supressor than the datasheet I posted earlier:
http://www.onsemi.com/pub_link/Collateral/MR2535L-D.PDF
