Venmo isn't instant. Send yourself more than 15k. If that goes through, try to send it back.

Sure, the row in the database is instant though.

I agree it is a feature for a bank to be able to undo mistakes. The problem happens when a bank commits the mistakes and refuses to acknowledge it.

This is akin to all of us submitting to google and gmail, its great unless our account gets flagged by an algorithm by mistake. No mistake, i would be screwed.