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 the early version of 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. Since the 2020 revision of Trader, however, the rule no longer applies to it, since the revised version of Trader does not prevent you from gaining a card.
 List of cards that can invoke the Blue dog rule
If a chosen Supply pile is empty, or a split pile with no more copies of the desired card remaining, this also hinders gaining. Trying to gain from an empty pile requires some shenanigans, such as triggering a Sheepdog, played Way of the Mouse as Lurker, to trash the last copy of the card you were trying to gain.
 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.