I came across an interesting but cryptic MySQL error message today that took me a while to figure out.
After creating a bunch of InnoDB tables on this particular server, I tried to create some indexes and insert some data. Both types of operations failed with an error message that looked like this:
ERROR 1025 (HY000): Error on rename of './dbname/tablname' to './dbname/#sql2-12bb-9474d' (errno: -1)
After some looking around I came across this amusing bug report, which explained that this message could be due to the innodb_force_recovery setting being set to a non-zero value.
Sure enough, running SHOW VARIABLES LIKE 'innodb_force_recovery'; revealed that it was set to '4' instead of '0'. I now have a sys-admin to kill.