Ford - Focus :: 2002 - Stalls In Park After Battery / Alternator Replacement

Last week, I brought my 2002 Ford Focus (automatic transmission) to the mechanic to have an oil change and to have the motor mounts and cabin air filter replaced. Before I brought the car in, it was fairly loud at idle--which had been going on for a couple of years and which I attributed to the worn motor mounts--but had otherwise been running fine.

When I went to pick it up after the repairs, the car would not start. The mechanic ran some tests and told me that the alternator was overcharging the battery. The battery could no longer hold a charge, so both the alternator and the battery needed to be replaced, which the mechanic did.

I thought at first that everything was OK after the battery and alternator replacement. Because I almost always turn my car off immediately after putting it in park, I didn't realize what was happening and that it was a problem... but in retrospect, I realize that ever since I got the car back from this second set of repairs, the battery light has flashed on during the split second between when I shift to park and when I turn the ignition off.

Yesterday, I took my car back to the mechanic because it stalled when I was in the parking lot at work. (I realize now that it must have happened just after I shifted gears, probably into park.) Since that was the first time it had actually properly stalled--rather than just a quick battery light flash just before I turned the car off--I had not yet made the connection between the stalling and the shifting to park, so all I told the mechanic is that it stalled on me when I was slowing to a stop. I'm pretty sure the first time it stalled was the first time I let it idle for more than a half-second in park before turning off the ignition.

He kept my car for a full day and ran a lot of tests on it. He was unable to make the car stall during multiple test drives (though, to be fair, I hadn't told him that the trigger for making it stall was shifting to park because I didn't realize it myself yet). There were no engine codes, the check engine light was never on, the fuel pressure is OK, and the throttle body position sensor, the idle air control valve, the ignition system, and the fuel system were giving no abnormal readings. When I asked, he told me that they did perform an idle-relearn procedure after replacing the battery and alternator. He returned the car to me and told me to come back if it stalled again.

Tonight, when I got the car home, it started to die immediately when I put it into park (the headlights dimmed and the battery light came on). I was able to reproduce this a second time as well. Fortunately, it does start back up just fine.

I plan to take the car in for repairs again, but I'm wondering if I should go back to the same mechanic. My questions are:

1) What is likely to be causing the problem?
2) Is it something that is likely to have been caused by incompetence when installing the new battery and alternator or doing other repairs, or could it be a coincidence (I was willing to believe the first coincidence of the battery and alternator dying while the car was at the repair shop, but I'm having difficulty believing a second coincidence)?
3) Whether or not the problem is likely to be the mechanic's fault, is it something that a thorough check on their part should have discovered?