I don't know as this counts as a bug, per se, but this is the most appropriate forum I could find for my question about the Psydex.

A number of pokemon are listed in GSC as being findable by Headbutt. There are often two percentages given for a particular pokemon at a particular location and time of day, distinguised by "(Common)" or "(Rare)" being appended to the end of the route name. Since the percentages for a particular location, time of day, and rarity add up to 100%, I'm presuming that sometimes, when you do a headbutt, it rolls on the 'common' table, and sometimes on the 'rare' table. What determines which table it rolls on, though?