I note TSB has had some problems with their website, but the bit that interested me was that users were met with an Unexpected Error and I have to wonder if there's any other sort of error. Would you release software that's full of expected errors? I could just imagine an error along the lines of: Expected error, we just couldn't be arsed to fix it.
It reminds me of a bit of mainframe software I used to work on called JES2. It used to have an error that simply said Something Wrong, which is hardly a great start when it comes to debugging the error.
Error messages should at least point the user to a potential cause. It doesn't necessarily mean the user can fix it but it might hint at something they could try.