A mischievous student on a dig can easily send the head archaeologist into a panic simply by burying a crisp packet in a section of earth that had been firmly identified as mediaeval.
The presence of something modern would mean the earth must have been churned up, thereby invalidating the carefully constructed pattern of dates, and potentially ruining years of research.
So spare a thought for those who had tentatively – but excitedly – announced the discovery of an “amazing” stone circle in Aberdeenshire and speculated that it was a relic of the Bronze Age.
After publicity about the find, the farm’s former owner came forward to reveal that, actually, he had built the ‘replica’ in the 1990s, so it was more like 25 years old, rather than 4,500.
To be fair to the experts, they had realised the circle was slightly unusual and commented on how uncommon it was for such monuments to go unnoticed for such a long time.
And the stones themselves are almost certainly quite ancient.