Here's a little tip we found out about the hard way.

We made a super giant TNT explosion in Minecraft, accidentally. We were playing with WorldEdit commands, and we made a huge underground TNT ball. BOOOOOOM. To our surprise (though in retrospect this is obvious) it was immediately ignited by underground lava. The explosion was so huge that it crashed Minecraft. We thought we would have to abandon the world.

But then one of us had an idea. We opened the world in MCEdit. It turns out that when you close a Minecraft world, any explosions going on are saved as "entities" (like chickens). In MCEdit we could see our huge cloud of explosion entities (in red) and we just selected them and deleted them. [To do this, make a selection normally (with the selection box), then in the menu choose "Delete Entities."]



We also had a lot of unexploded TNT left over which would surely start the whole problem all over again, since there was still lots of lava around. We selected the chunks with the TNT in them and asked MCEdit to delete them from the world. That caused MInecraft to regenerate those chunks without the TNT in them the next time we opened the world in Minecraft. [To do this, make a selection, then choose the "Chunk Control" icon on the bottom picture menu thing, then choose "Delete" from the menu. Do NOT choose "Prune" - it will cause all the OTHER chunks in the world to be deleted.]



World fixed. Lesson learned.

Now anytime we have a problem with a world we open it in MCEdit and see what we can do to fix it there. Hope that's helpful to people!

EDIT: Screenshots added. (Okay, those aren't actually explosions, they're sheep; but you get the idea. We didn't actually delete them either!)

woodspeople