Advanced Dungeons & Dragons (2nd Edition) introduced a concept of "wild magic" in an effort to make magical spells more interesting and less predictable. Wild Magic was emphasized in Tome of Magic with the Wild Mage specialist. Wild Magic spells usually were powerful, but often had unintended random side-effects.
While intriguing, the process was unusually burdensome to the Dungeon Master who was expected to come up with scores of "random" strange effects. Wild Magic often stagnated in a campaign once a particular Dungeon Master's imaginary repertoire of strange effects was exhausted.
Various fan-made "net books" circulated to alleviate the beleaguered Dungeon Master's efforts. These were typically little more than lengthy lists of random magical side effects. These were exactly what the Dungeon Master needed.