Care must be taken not to spread deep-sea creatures around the world during exploration of the remote ocean floor.
Scientists using the famous Alvin sub say the vehicle picked up limpets from a depth of almost 3,000m and inadvertently transferred them alive to another location more than 600km away.
It is surprising because the animals had to cope with huge pressure changes as Alvin conducted its dives.
The researchers report the event in the journal Conservation Biology.
Spreading organisms artificially beyond their range in this way could have damaging effects on marine ecosystems, they warn, either by introducing competitors or even disease.
The team urges other deep-sea explorers to exercise extreme caution, and to assume hardy stowaways could be hanging on to their vehicles.
The curious case of the limpets is described by Janet Voight, from the Field Museum of Natural History in Chicago.
She and colleagues were studying lifeforms living around hydrothermal vents off the north-west coast of the United States.
Such vents are among the key study targets of modern oceanographic science – places where amazing collections of animals and other organisms thrive in mineral-rich, hot waters that gush up from volcanic cracks in the seabed.
Indeed, it was Alvin that first discovered these ecosystems in 1977.
On one dive to a depth of 2.7km on the Gorda Ridge, the team gathered various specimens with the submersible’s sampling tools.
The group then climbed back up the water column and moved Alvin, via its support ship, to a new dive location 600km to the north, on the Juan De Fuca Ridge, to take yet more specimens.
When the scientists examined their haul back in the lab, they found examples of a limpet (Lepetodrilus gordensis) thought to live only on the Gorda Ridge existing also on the Juan De Fuca Ridge. The team thought it had discovered an entirely new population.
But doubts started to creep in when the researchers realised conditions at the second site could not have met the limpets’ nutritional requirements.
Chemical analysis then confirmed the two populations were in fact one – Alvin, despite being cleaned en route to the second dive, had carried stowaways in its sampling gear.
“It remains a mystery, but we suspect we didn’t fully clean the suction sampler,” Dr Voight told BBC News.
“Perhaps they were in a little crack somewhere. The hose on the suction sampler looks much like the hose on your vacuum cleaner, and perhaps they were hiding in the corrugations. We were late coming up from the dive and it was a bit dark – maybe they just weren’t seen.
