I don't think its OK to screw up, but in the event that you do screw up, step up and take accountability for it. This all boils down to being honest with your customers.
Of course, to screw up is to err (by definition), but I meant it's OK in the sense of it not being the end of the world. Your humour is an illustration of this. It's possible to get hung up on avoiding mistakes at all costs - indeed, at greater expense than the mistakes themselves.