Be Careful with Not In Program as a Program Status in Choice Steps
Be aware that using "Not in Program" as a Program Status for a choice step in Flow, may lead to unexpected results.
We have set up an invite flow for a webinar so that on the first run leads get email "a" via choice step (as not a member of the program) and on the 2nd run they get email "b" (default). As part of the flow leads are added to the program based on whether the email was actually sent or not - so after the first run they are a member of the program.

Everyone got email "b" on the first pass.
Turns out the default choice is applied - I still don't quite understand why Marketo doesn't see them as Not In Program but it was advised by Support.
In this use case it would be better to apply a Not Was Sent email choice step i.e. Not Was Sent email "a", send email "a"; default to "b".