First off, I still write checks. If you want to electronically put money into my account, fine, but to get any out you need a piece of paper from me.
Offhand, I can think of 3 cases where the bank or CU made an error and all were resolved in my favor. One was a teller data-entry error that she caught right after she hit the enter-key, so that was fixed immediately. I still have a statement somewhere that shows a $1.5M deposit into my account, followed by a correction, though. One was a withdrawal from my credit union account that I know I didn't make/authorize and after a couple of days for them to "investigate" they found the error and fixed. The third was the teller mis-keying my account number on a deposit, which I'd have caught sooner if I requested a balance at the time since the PIN wouldn't have worked. Again, one phone call and they fixed that problem, and from that point on, I ask for the balance to make sure they have the number entered correctly.

Offhand, I can think of 3 cases where the bank or CU made an error and all were resolved in my favor. One was a teller data-entry error that she caught right after she hit the enter-key, so that was fixed immediately. I still have a statement somewhere that shows a $1.5M deposit into my account, followed by a correction, though. One was a withdrawal from my credit union account that I know I didn't make/authorize and after a couple of days for them to "investigate" they found the error and fixed. The third was the teller mis-keying my account number on a deposit, which I'd have caught sooner if I requested a balance at the time since the PIN wouldn't have worked. Again, one phone call and they fixed that problem, and from that point on, I ask for the balance to make sure they have the number entered correctly.