Conditional evolution is a glitch effect which refers to the unexpected evolutions of Pokémon after battle, or outside of battle.
These effects occur in both Pokémon Red, Blue, and Yellow and Pokémon Gold, Silver and Crystal.
In Generation I
A non-freezing Super Glitch (containing a 0x50 sub-tile on the saved copy of the screen) will give the player the expanded party, and will likely cause unexpected evolutions if the Super Glitch effects are activated in a Pokémon battle and the battle finishes.
In Generation II
The Ilsty and ? glitch involves viewing the hex 0x1C Glitch Unown in the Unown listing.
This causes a Pokémon called "ILSTY I PkPkPkHpPOKé ÄMn POKé" to evolve into a Forretress, followed by a Graveler named "?" evolving into Golem.
Additionally, specific glitch Pokédex categories are capable of causing conditional evolution, such as Pokédex sorting 0x7C in Gold/Silver or Pokédex sorting 0xF3 in Crystal.
Ilsty and ?: