Earlier quoted context omitted.
The specifics of it are dependent on the bank, but it can be more insidious than even your example. Given a balance of $100 and 4 transactions of $20, $20, $20, $90 you can either be charged one, or three overdraft fees depending on the order in which your bank deducts those from your account. Basically, ordering from largest to smallest will cause the most number of overdraft fees.
I'm going to re learn the lyrics to Oh Canada and sing them joyously and thankfully... Our banking system gets a lot of ire but dear Gawd.
The benefit of US banking despite lax regulation was that there's a ton of different banks and credit unions to choose from. Some are great and some are shit. Literally hundreds to pick from. Canada there was like 5 big banks, a large credit union, and then depending on your area there was maybe perhaps a small community credit union.