Imagine you are expecting a pay-rise at work, and then you open your wage slip and find that instead of the £3.5k raise you were expecting you got an increase of £12.5k per year.
This is obviously a mistake, and one that should be easily noticed at the next salary review...
Would you own up ASAP? I think the key phrase here is "plausible deniability" - if it transpired you knew of the mistake and didn't say anything then that would probably be construed as gross misconduct.
Not quite sure if I could pull off the plausible deniability thing... my boss certainly knows I am not hanging on for pay-day and accounting for every penny, and I could stop opening my payslips from now on so if challenged I could show them unopened "except for the one I opened last night to verify I was getting too much". Probably worth the risk - if I was suddenly asked to leave they'd be fucked anyway so the worst they could really do is get me to pay it back.