It's not the dirty stuff you are always thinking about, but instead some geek stuff: you can have an additional cron coming out of nowhere by having a cronjob set-up like this:
30 02 * * * ~/src/perl/ftpstats/export_general_cdrs.pl -c 123456
Long story short, this cronjob was executed twice Sunday night, because of the time change (2006-10-29 02:00 repeats twice in the night); more details here. This may be elementary to some, but it just occured to me today. In April/March it's even worse, because this cron would be completelly skipped.