While I believe a reasoning similar to the image that Karsten posted, as generations go by we start to forget the intelligence of people in the past lost through time. One of the best examples is the Egyptian Pyramids; while accounts are made of the masses of slaves, scientists are still stumped how they could have worked to create such massive structures -- with particular concerns toward the weight of each block -- in such a short time frame.
Though obviously the Mesoamerican calendar had to stop somewhere, it doesn't mean there wasn't reasoning for stopping where they did, though it doesn't mean the end of the world of course. If I started counting with the intention of reaching a very high number, it's likely I would eventually choose ahead of time where I was going to stop ahead of times rather than stopping in my tracks. I would, for instance, stop at 1,000,000 or 500,000 or 987,654,321 or even 314,159,268 (the first nine digits of pi,) but not some totally off-the-wall number.
With all of that said, there were several groups of people, mainly guided by cult or religious leaders who sealed (and accidentally trapped) themselves in caves or drank poison a few days before January 1, 2000 so that they could avoid the chaos they had expected to come. I'm sure there will be several similar instances next year.
