crash on RPi with subsequent boot failure

[Dan Raymond]

I've been running r273702 on my Raspberry Pi B+ for the last 10 days or 
so.  Today while I was using an SSH session it suddenly stopped 
responding.  I power cycled and it wouldn't boot (I tried three times 
and it failed every time).  The following error was displayed on the 
console:

start = 0, len = 540, fs = /
panic: ffs_alloccg: map corrupted
KDB: enter: panic
[ thread pid 610 tid 100052 ]
Stopped at      $d:     ldrb    r15, [r15, r15, ror r15]!
db>

I was able to boot after re-imaging the SD card and I am now working on 
restoring all my previous settings (which is a lengthy task since 
building ports is very slow on the RPi).  Any ideas what happened and 
what I can do to prevent it from happening again?