Paramedics were called to a man who had fallen off his bicycle in Queensway, Hemel Hempstead.
The accident happened during rush hour - just before 8.30am - on Monday.
But when the ambulance crew arrived it was discovered that the cyclist had suffered no injuries and did not require treatment.
It caused some delay to motorists heading to work as the parked ambulance, which had its blue lights flashing, had blocked one side of the carriageway.