Two men were seen "peering over the edge of a precipice" near Bantham, Devon while a friend took their photo, said Maya Plass, who lives nearby.
They were standing under another section of cliff that is showing signs of severe erosion.
Ms Plass, a marine biologist, said the men were "completely negligent" and warned other walkers of the dangers.
Ms Plass, who walks the cliffs regularly, said the section next to the coast path was slowly falling away.
"Every time I go there it seems the fissure has grown and the cliff has moved closer to falling to the beach," she said.
"It could go at any time because the hot weather will expand air pockets in the cliff and send it off."
She spoke to one of the men taking pictures, but he said they were "fine", she said.
"It is not fine. There are some pictures which are not worth the risk of taking.
"Putting individuals at risk like this and potential coast guards is completely negligent."
Bernard Parker, chairman of the South Devon Group of The Ramblers, said erosion of some cliffs was "constant" and visitors were often unaware of the dangers.
"You can see when you walk the coast path many places where cracks have become evident," he said.
"It is quite concerning and I don't think the general public are aware of the seriousness of it.
"There are some paths that have been moved back at least five metres over the last 10 years because of the erosion."
Sightseers are being warned they are putting their lives in danger by standing under an unstable cliff.