That you intentionally overdrafted your account because you "knew" they posted transactions in a certain order.
After all the ONLY way you can get overdraft fees is if you overdraft your account. If you don't overdraft your account it doesn't matter how they post them. There is no way they can turn a positive account to an overdrafted account by changing the posting order.
I would bet that you were notified. Either when you originally opened the account, or were sent a change of terms at some point. Yes those pages of "fine print" are actually important.
Now, here is your other issue. As of last summer you had to "opt-in" to the banks overddraft protection. During this time the banks sent out notifications and unless you "opted-in", any point of sale transaction(Debit Card/ATM) would be declined if you do not have enough money available in your account to cover it. So either you did "opt-in" or you wrote checks/ACH transactions that are not covered under the new regulations. If in fact you did "opt-in" your next stop should be to go to them and "opt-out".
By the way this is pretty much the standard at most other banks and credit unions as well. So don't think changing banks is going to solve your problem. The only sure way to avoid overdraft fees is to properly manage your account and not overdraft.