@JeremyECrawford Cast Hold Person on a humanoid that's concentrating on Fly. Paralyzed.

Does it fall out of the sky?

— John Appleton (@jaa0109) July 28, 2016

If you paralyze the spellcaster, the spell ends, since being incapacitated ends concentration.

— Jeremy Crawford (@JeremyECrawford) July 18, 2017