Payment rails that have to work when the connection does not. Most of the engineering is not the happy path; it is what the system can prove about a transaction it never got a receipt for.
Identity that survives a person changing their phone number — which in practice means identity that is not the phone number, whatever the table says.
A habit of writing down what I expect before I measure it. Half the time the expectation turns out to be the interesting artefact, and I would not have noticed if I had only kept the number.