While using a command block structure (detailed below), my single player world experienced server-side lag (eg fire animation remained after block destroyed, mobs frozen, player still able to move and place blocks).

I relogged to re-sync my client and server, and when trying to re-enter the world, the client bumped me back to title screen.  When I tried again, the client crashed.  Upon re-launch of the client, I was greeted with the below crash reports (two as it happened twice).  It should be noted that these took a while to pop up, about 10 minutes after the actual crash occurred.

The command blocks:

The general idea of the command blocks was to create "NyanBats".  Basically, a 7 tall structure of rainbow coloured wool is cloned two blocks below an invisible, invulnerable bat.

The exact structure consists of a /setblock redstone block clock (on replace, not destroy), connected to a command block to keep the bats away (/kill @e[type=Bat,r=100), and the actual clone blocks command (/execute @e[type=Bat] ~ ~ ~ /clone -298 56 -537 -298 61 -537 ~ ~-8 ~ masked). [edit /clone command originally, and erroneously, of type "keep", not "masked"].

The crash logs seem to indicate that the crashes occurred AFTER I logged out, and when this clone command was being executed.

Also, at the time, I was testing the /kill 'safety' system, clearing out excess wool near the command blocks with fire, and building a lava moat to keep mobs out.

Edit:

On further testing, the incorrect clone type (keep) was not at fault.  I was able to replicate the crash by again causing server-side lag (this time by a decent number of tile entities, and attempting to place blocks).  This leads me to think that this might be an issue with the way the @e selector processes through large amounts of entities to locate specific ones, along with the chunks being unloaded upon exiting the game.  Just an idea though, no real hard evidence.

Also, I've uploaded the original reports under A, and the second crash reports under B.  There is an interesting "Exception Ticking World" crash that occurred, although I only had my client crash once this time.

Feel free to ask for more details.