Mistakes happen, imagine a banker who has to deal with multiple requests a day for funds.
Its easy to catch sure but after 100s over time I would not be surprised if people stop reading the warnings and go through the motions each time. Add on a poor night's sleep, or multi-tasking I can see it happening.
I'm not a banker myself but think having a final step popup when paying someone more than their balance to manually type out the exact payment amount could stop this happening again accidentally. (With no auto-fill max button of course!)