Can you cast a reaction spell on your turn?
You sure can!
Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius also has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.
The order of events alluded to by Sage Advice would be:
- Cornelius begins casting Fireball
- Cornelius' opponent begins casting Counterspell
- Cornelius casts Counterspell, preventing his opponent's spell
- Cornelius finishes casting Fireball
- There is much fire
So far as I've been able to tell, it seems that the example given in Sage Advice would result in Cornelius essentially casting Fireball and Counterspell simultaneously.
By extension of the same mechanic, a caster could cast Shield in response to an Opportunity Attack by an opponent with the Mage Slayer feat. A Warlock could use Hellish Rebuke in the same circumstance.
This doesn't seem correct, at first glance, as it means that a caster would be able to do multiple complex somatic components, or to incant arbitrarily many verbal components simultaneously.
This seems strange enough that it may be that the Sage Advice column has overlooked a rule preventing this. Alternatively, they may have just skipped an implicit "but you give up the fireball by doing this, of course".
In short, can a character effectively and successfully cast two spells at the same time?