Not exactly what the edge case is, but my hypothesis is for attempting to create a PR from a recently dethorned (i.e. I recently changed the default branch from develop to master) branch onto any branch of the same repo (forks don't count).
At least it's just a warning and not an actual error. Like all warnings, you can ignore it.
Aatube@kbin.social to 
Not exactly what the edge case is, but my hypothesis is for attempting to create a PR from a recently dethorned (i.e. I recently changed the default branch from develop to master) branch onto any branch of the same repo (forks don't count).
At least it's just a warning and not an actual error. Like all warnings, you can ignore it.
God bless
"Don't use GitHub" comments incoming...
It's been two months sir