For years I regularly had Barclays block my card for things like buying the same plane tickets to fly see my family that I've done every year for more than a decade, from the same airline, to paying for groceries from the same online merchant I've paid for groceries at roughly the same amount every week for a couple of years.
The one instance I've had of fraud, on the other hand, their system didn't catch at all:
Someone paid two credit card bills of 4k each at a bank I've never before transferred money to, for accounts not in my name. I very rarely make big transfers, and certainly have never in my life paid money to a card account not in my name. You'd think that would have set off some red flags and the usual automated call telling me my card has been blocked until I confirm some transactions over the phone. But no.
Now, they were quick to take action and moved the money back into my account near immediately (with a stern warning that if I didn't sign the relevant documents in time to confirm what I had stated to them, the money would be taken back) so I wasn't inconvenienced that much. But having regular, routine payments being blocked semi-regularly while two unusually large transactions to a bank I never transfer to went straight through without additional check made me question what the point of their fraud checks is.
I've been told I've failed an identity check while on a call from a branch, when that call had been started by a member of staff from that branch who had identified themselves and was standing next to me at the time, while I was in possession of more photo ID and proof of current address than I've ever needed to open any account in my life. And all that member of staff could do afterwards was apologize and share a half-hearted laugh with me as I left.
This happened while signing up for a new service, and ironically I was in the branch in the first place because I'd been unable to answer a security question on a call from my own phone. That was because the only two answers I was allowed to choose were both obviously incorrect, because the subject of that question was also the reason for my call, but again the person on the phone apparently wasn't allowed to accept that and continue.
I was on holiday for a long weekend, Thursday-Tuesday. It was Friday evening when I called them.
Came home, got a day job, and my card got flagged for 'suspicious activity' when I started following the same pattern every day of buying a train ticket and lunch from the same shop. If that's suspicious, there must be some really unimaginative fraudsters out there.
The handling of accounts involved in suspected fraud is very frustrating for the account holder because the bank becomes much less communicative.
In my experience, they do what they want.
Another anecdotal data point with a different UK bank: Recently an acquaintance has found themselves frequently locked out of their account because (according to their fraud team) someone is failing authentication challenges on the phone. My acquaintance has had to visit a branch multiple times to verify their identity and get passwords changed.
So even though someone can't prove who they are, they're locking my acquaintance out. How's that fair?
The kicker was one phone call where (after having discovered the account was locked... again...) the fraud team on the phone sounded incredulous, asking why we hadn't gone in to branch like they asked "last night"? Less than 30 seconds later they noticed last night's call was marked suspicious... that authentication failed... and that that phone call had triggered the lock the account.
The worst part is that no "human" will take your case on. They read from a script and aren't allowed to empathise. No "I'm really sorry, I can see why you're upset" or even "If what you're saying is true, I wouldn't be happy either."
Error is everywhere. The important thing is how people and systems detect, assign and recover from it.
Nowadays I do all foreign transfers via HiFX (many other banks have a decent system for international transfers so I could set up another account, but I stick to Barclays for my actual bank accounts because the aracane nature of UK credit scoring means that as a non-citizen not on the electoral roll, having a bank account that's been open for a decade and a half is worth a lot in financial flexibility; and while they annoy me, they don't annoy me enough to go through the hassle of operate another account and let it "age" for a few years)