For quite some time, whenever I returned from lunch my MacBook Air was frozen and the fans were running full throttle. It seemed to always happen whenever I was away for more than ~40 minutes or so and the screensaver kicked in. I started investigating on the issue. One of the first suspects was Spotlight and I thought it might have to do with either an external hard drive or Mailsmith indexing. Since the latter was turned off in Spotlight, I also turned off indexing for the external drive. After a few days, the freeze still happened. I tried rebuilding the Spotlight index a few times and also deleted the /.Spotlight-V100 folder, both without luck. Further investigation led me to some suspicious entries in the console logs and after closer observation, these repeated everytime before the crash:
`ps_allocate_cluster - hi_wat_alert
mds: open index shadow error`
I couldn't get any further on these entries. One day I found out that the freeze was due to running out of disk space at some point and somehow Spotlight must have caused this to happen.
I decided to reinstall Mountain Lion to avoid further troubles and indeed, this fixed the problem. Since then I haven't seen this freeze happening again and everything runs as expected again.