Comment by kukkeliskuu

I have worked with core systems in several financial institutions, as well as built several event sourcing production systems used as the core platform. One of these event sourcing systems was actually providing real consistency guarantees.

Based on my experience, I would recommend against using bank systems as an example of event sourcing, because they are actually much more complex than what people typically mean when they talk about event sourcing systems.

Bank systems cannot use normal event sourcing exactly because of the problem I describe. They have various other processes to have sufficiently probable consistency (needed by the bank statements for example), such as "reconciliation".

Even those do not actually "guarantee" anything, but you need tertiary manual process to fix any inconsistencies (on some days after the transaction). They also have timeouts agreed between banks to eventually resolve any inconsistencies related to cross-bank payments over several weeks.

In practice this means that the bank statements for source account and target account may actually be inconsistent with each other, although these are so rare that most people never encounter them.