@JeremyECrawford I'm sure someone has asked this but I couldn't find a definitive solution. Can you cast feather fall in time with 1 reaction, considering the "Xanathar's guide" rules on falling, where you fall 500ft immediately. — Gianni Pratico (@Wulfgar15) May 14, 2019

Feather fall, like many things in D&D, creates an exception: it lets you cast a spell as a reaction to slow someone's fall. If a spell or other exceptional thing in the game says it does something, that's what it does. Also, any rule in "Xanathar's Guide" is optional. #DnD https://t.co/ELqT7HFv0d — Jeremy Crawford (@JeremyECrawford) May 14, 2019

As we used to say in older editions, "specific trumps general". The specific case of falling with Feather Fall trumps the general rules for falling. — Wonton (@Wonton_) May 14, 2019

In D&D, a specific rule always beats a general rule if they disagree with each other. For more information about that principle, take a look at the section "Specific Beats General" in the "Player's Handbook" (p. 7). #DnD https://t.co/1eTsemrqPO — Jeremy Crawford (@JeremyECrawford) May 14, 2019

"Also, any rule in XGtE I optional." <3 Say it again. Say it louder. (This is of course above and beyond the "optional" nature of all the rules.) — Dave Williams (@DaveWil33) May 14, 2019

The official rules of D&D are in the 3 core books. Those rules appear in a shorter form in products like the Starter Set and the free Basic Rules. Any rules that appear elsewhere are optional. Your DM decides how to use those rules in service to your group's fun. #DnD https://t.co/hTmUQnFORi — Jeremy Crawford (@JeremyECrawford) May 14, 2019