A month or two ago I heard for the first time that all the planets were going to align on December 21, 2012, and that the resulting tidal forces would unleash disastrous floods and possibly earthquakes.
This, I thought, sounded rather cool. Not the floods and earthquakes, of course -- but the idea of a planetary alignment sounded interesting. There wouldn't actually be any flooding or earthquakes, because the positions of the other planets have nothing to do with that sort of thing (they're too far away to affect our tides).
Wondering which planets were going to be "aligned" (i.e., in conjunction or opposition), I fired up SkyGazer, a planetarium program that came with my college astronomy textbook, and punched in 2012-12-21, and got something of a surprise: On that date there isn't going to be any sort of alignment at all.
Solar System Live gives this result for Dec. 21, 2012:
Granted, Pluto is going to be nearly at conjunction, but that happens every 367 days and just means it'll be behind the sun. So, not only was the part about global calamity wrong, but the event that was supposed to cause said calamity isn't even going to happen in the first place.
As far as I can tell from a bit of googling, the entire erroneous alignment story somehow arose from the fact that the Mayan Long Count calendar reaches the end of its cycle on that date, something that (according to Wikipedia) appears to happen roughly every 400 years or so and doesn't actually mean much unless you want to use it as an excuse for a massive Mayan New Year party.