This is what I ran into on a job today...
THHN ran open spliced onto a piece of extension cord, spliced into a sliced open piece of Romex, true engineering at work.
Whew! At first I was worried, but then I saw that strain-relief knot on the extension cord! I guess everything is OK after all!
I like the knot too. Bonus points for the single strands put together with electrical tape. Very professional.
And look, they extended the wire without ANY junction box!