RBF Is a Safety Net You Have to Set Up in Advance

One of the more counterintuitive things about Bitcoin fee mechanics is that the fix for an underpriced transaction, RBF, isn't something you can apply retroactively. It has to be signaled at the moment you broadcast the original transaction, as a flag most wallets surface as a toggle you'll never notice until you need it.

This creates a specific kind of frustration: the exact situation where you'd want RBF (a transaction stuck due to a low fee) is also the situation where it's too late to enable it, if you didn't check the setting beforehand.

The practical takeaway is simple but easy to overlook, if you're sending anything time-sensitive, check that RBF is enabled before you send, not after you're already watching an unconfirmed transaction sit in the mempool.