Mana Abilities

In order to play spells in Magic, you usually have to pay for them with mana, usually generated by using some kind of "mana ability." Island has a mana ability, Black Lotus has a mana ability, Skirge Familiar has a mana ability. In 1997, tourney hopeful David Mills was disqualified from the finals of Pro Tour: Los Angeles for announcing spells and then using the mana abilities to pay for them after, which isn't an uncommon practice. This led to history's only Magic riot, as a group of furious neckbeards stormed the stage during the awards ceremony and demanded justice for Mills. The judges relented and awarded Mills a second-place prize, and proceeded to change the rules so that it became legal to activate a mana ability while paying costs. Now the rules state that a player can't do anything in response to a mana ability, so nothing can disrupt said mana ability once it's started.

This brings us to Caged Sun, a card that doesn't really work. As hinted at with the prior examples, all mana abilities are activated (i.e. pay cost: do effect) with the sole exception of Caged Sun, which is triggered (i.e. when trigger, then effect.) The rulings on Caged Sun state that it does have a mana ability nonetheless, as otherwise this would create some awkwardness when playing spells with the Sun in play. In summary, Caged Sun has a mana ability that adds mana to your mana pool whenever a land adds mana to your mana pool. Okay, not too complex. The rules start to break down when Caged Sun itself becomes a land, however. This can be accomplished by combining a few cards...

March of the Machines makes all artifacts into creatures, so Caged Sun is now an artifact creature. (You starting to notice how often this card comes up in these articles?)
Artificial Evolution or Xenograft make your creatures into a chosen creature type, so Caged Sun can become an "Artifact Creature - Saproling."
Life and Limb makes all saprolings into lands, so Caged Sun is at last an "Artifact Creature Land - Saproling Forest."

So at this point, Caged Sun has a mana ability that effectively reads "Whenever Caged Sun adds mana to your mana pool, add one mana of that colour to your mana pool." Neither player is allowed to do anything until this loop completes, so the players sit there helplessly as the game repeats itself ad nauseum. At this point, the game is nothing less than a draw.

Caged Sun is a fun example of the problems potentially caused by type-changing effects, and also the looming threat of infinite loops. An infamous type-change conflict exists by combining Humility and Opalescence, which causes Humility to lose the ability that causes it to lose all abilities. But wait... then that would mean... yeah, things get kind of confused.