There have been at least 2 occasions where I have given accidental sex changes to both humans and 'mons across the entire server.
The first time was during the introduction of HG/SS overworld sprites to replace the GBA ones, and the ID for the Old Ladies got mixed up with the one for the Old Men.
The second time was when we updated the gender icons; I gave them to Shu to add to the client files but forgot to put them in the visual order they were originally. (Males on the left and Females on the right).
This meant that post-update every 'mon on the server was [visually] displaying the incorrect gender for a day or two.
This led to both some very angry and also hilarious bug reports.