My game crashes everytime I try and load my saved world.

Any save game I try and load from the past 6 months crashes. But if I go back to a save from 6 months ago I can play. Something I built is causing the crash, I'm guessing something based on redstone, but that's a huge guess.

I was also able to find a save where I wasn't in my base. I was able to load up the game, then I traveled through the nether and enter the portal to my base, then the game crashed.

Added by Auldrick
There is evidence that worlds created on earlier releases (prior to 1.0?) are getting upgraded incorrectly, corrupting them and causing them to crash. The crash seems to occur when the player is in particular locations, perhaps near an unknown block or entity that was incorrectly upgraded. Because many players stay in one place for an extended time, they may be near the problem location most of the time, and in that case the world crashes almost immediately after loading.

Currently, the only known workaround is to leave the Beta, revert to release 1.2.9, and restore a backup copy of the world. Reverting requires uninstalling Minecraft after leaving the Beta. This will delete all worlds and return all settings to the defaults. Be sure to save a backup copy from 1.2.9 before you open a world in 1.2.10, and keep those backups until 1.2.10 is out of Beta.