I agree with all said, but want to add a little point about UX or bad experience... when you handle money... UX is the "last" of your priority, the back fire when you lose other people money is SO bad for your application that throw all the UX to the garbage.

Focus on "don't" lose money first, and then add all the shitty lights, remember... we are competing with banks... worst UX in the world.

Actually there were locks in place for these, but the bug was much less trivial: it was timing between the time it took to save the new tx to the dB and the balance compute from dB. There were (keyed per wallet) locks all over the place