Blue dog rule
The "blue dog rule" is a ruling by Donald X on what to do when an ability has an effect based on a card it's gaining, but the card to-be-gained is never actually gained for one reason or another. In this case, the ability has no effect, since it did not gain a card.
The archetypal example of this rule is the interaction between Ironworks and Trader. If you would gain a card with Ironworks, and then reveal Trader to gain a Silver instead, you never actually gained anything with Ironworks, so Ironworks fails to give a bonus effect based on the type of the card gained.
It is called the "blue dog" rule because when Hinterlands was first released, Donald X posted a bizarre anecdote about walking a blue dog to try to explain the interaction.
 List of cards that can invoke the Blue dog rule
If a chosen Supply pile is empty, this also hinders gaining.
 Gain-effect cards
This applies to any cards with an "If you do" or similar clause preceding an effect dependent on a card gain (such as Ritual), and it's usually quite clear that nothing happens, because of this phrasing. However, a few cards have no such phrasing, leading to potential ambiguity; this ruling was given specifically for these kinds of cards:
Donald X.'s original quote:
While the quote provided the name for the ruling, afterward Donald X. changed his position on how the interaction should work to how it stands today.