what are some ways that a pacemaker could fail...

…in an otherwise young and healthy person (assuming the batteries had been checked, etc.)?

IANAD, but I would imagine software issues could be a problem. AFAIK, pacemakers are operated by a small computer chip and program, which regulates the function of the pacemaker. If the software (or hardware) was to crash, for some reason or other, then that could kill the pacemaker.

I suppose some sort of disconnection of a component, if the patient were to move or bump or injure themselves in a very odd way, could also disable the pacemaker, or at least shift the wiring or components such that it is ineffective in its purpose.

Don’t go making or receiving any cell phone calls !!!

(Right, everybody? Right?)

While you’re at it, don’t let your pacemaker intersect any cell antennas’ signals, which are already crisscrossing most metropolitan areas.

Don’t fly a kite in a lightning storm, either.
Wait a minute…

Why would a healthy person need a pacemaker?

Why would a healthy person need a pacemaker? **

Person with a congenital defect (a “block” or irregular heartbeat) but otherwise healthy (no illnesses or diseases or symptoms).

Metal detectors could cause a pacemaker to malfunction. I used to work for a pediatric cardiology division, and one day someone called me from a school, talking about how a patient of ours had been “wanded”, and since then had been complaining about not feeling well. According to the doctor I spoke with, yes this can be a big problem, and he had the patient come in so they could recalibrate the device. He had me type out a letter for the school, warning them to not use a metal detector on this patient.

Cite: http://www.appc1.va.gov/vasafety/docs/Metal-detctr.doc (sorry, .doc file)

There is some evidence that the gate-style detectors do not present a risk, but considering the low rate of incidents in the first link I gave, the 200 pacemakers checked might have been too low of a sample size to show any events. It might also vary by detector brand/age, and wand-style detectors were not tested.

The American Heart Association has a list of what devices do not present a risk, and which might be risky.

I’m assuming the OP meant “otherwise healthy” but I can’t think of why age or other illnesses/conditions might interfere with a pacemaker’s functioning.