How about accidentally accessing the wrong variable, that got passed into a shcll command run by some bad code on another machine?

The end result was that rather than limiting which files got transferred to Bloomberg's ftp site, every file that they had ever been sent was resent. This was rather more than the receiving machine was prepared for, leading to crashing the machine which they use for receiving updates from partners.

The most that I can say to defend myself is that none of the programs involved were originally written by me, nor was I responsible for the overall fragile architecture. (It was just my job to keep things going and improving.)

The worst that I can say about this mistake is that while strict.pm would have caught it, this was not the incident that convinced me to use strict.pm. (But I should hasten to note that this was back in 5.003 when foreach my $foo (@list) was an illegal syntax. Which made strict.pm much more annoying to use than it is now.)