This article has a talk page!This page details bugs of Pokémon Crystal.
Since Crystal is basically a souped-up version of Gold and Silver, all bugs that existed in the original games apply here as well unless specifically noted on that page.
Ho-Oh Bug#
In Crystal, Ho-Oh can normally only be encountered if you caught all three legendary beasts, and they were not traded from outside. However, the check for this only checks the first five letters of the player name, so even if the last two letters differ (and the name and ID are otherwise identical) the Pokémon will be treated as your own. The relevant code was never adjusted from the Japanese version, where player names were limited to a maximum of five characters.
Clair TM24 Bug#
In Crystal, after completing the quiz inside the shrine at Dragon's Den, Clair will give you the Rising Badge, and then, once you step out of the shrine, she will also give you TM24 (DragonBreath). If you cannot receive the TM at this point due to the bag being full (this requires sending 99 copies of TM24 as held items from another game, an extremely unlikely scenario), you can return to the gym at a later point and Clair will give it to you there instead.
However, by whiting out inside the shrine from having the last Pokémon faint to poison damage, after completing the quiz and receiving the Rising Badge but before leaving the shrine, it is possible to receive TM24 twice. Clair will give you the TM in her gym, and then she will appear a second time and give you the TM again if you return to the Dragon's Den and step in front of the shrine. This appears to be a simple oversight where the developers didn't consider there would be any other way to leave the shrine than by using the door, so the flags are not properly set to handle this situation.
Notably, in HeartGold & SoulSilver (which uses Crystal's version of the Dragon's Den), it is possible to leave the shrine at Dragon's Den by using Dig or Escape Rope, and in those games the flags are properly set for this case.
Heavy Ball Bug#
In Crystal only, the Heavy Ball does not load the correct weights for the three Pokémon Kadabra (64), Tauros (128) and Sunflora (192). Instead, the game loads garbage data that is interpreted as a massive weight, causing these three Pokémon to get a +40 bonus on catch rate if a Heavy Ball is thrown. The bug occurs due to an off-by-one error in the function that determines which ROM bank to load to retrieve the Pokémon's Pokédex data.