"Good bye" & "So long".
***
...
- Good-by, Danny
- Not "good-by", Ricky. Just "so long".
We'll be waiting for you"
-- Robert Heinlein, "The Door into Summer"
***

It turned out that "good bye" means something
final, inevitable - you name it - whereas
"so long" means you're gonna meet a person
again. None of native speakers gave
me that answer right away, all had to think
for a short while. Though I must say
that "good bye" is being used rarely.
"bye" or "bye-bye" - but not "good bye".
==
"Have a good one" - used very often.
Makes me laugh after watching one of
George Carlin's sketches with

- Have a good one!
- I already have a good one. Now I'm looking
for a longer one.