Having worked on several side projects that involve money, I'll bet most money-related computer systems are full of race conditions and other wacky behavior. There are so many gotchas. For example, just writing a simple "bank account" system/database, it's tricky to ensure in a performant way that concurrent transactions don't bring a user's balance below 0. If you're sending out money via ACH or something, try handling all the ways that can fail and need to be retried.

I trade regular stocks in my Chase account. A few times, it was down for maintenance after-hours, meaning I couldn't enqueue trades to execute the next day. Not a big deal, but doesn't inspire confidence.