You think you know where you are but you can’t find the mark that should be there. It’s a situation most yachtsmen find themselves in from time to time. It often turns out that an error has been made somewhere between the chart table and the helm. Variation or deviation was wrongly applied, BST was ignored, the tide was ebbing or flooding or leeway was ignored. Perhaps the GPS datum was wrong. You have a good scratch of the head, work out where you went wrong and plot a new position.
Sometimes, however, you cannot find the mark because some bugger has ripped it up and left it on the bank!