Almost every serious British mushroom poisoning follows the same pattern. Someone found something that looked like something they had eaten before. They were fairly sure. They cooked it anyway.
Fairly sure is not sure.
Why one source is never enough
A field guide photograph shows one specimen at one age in one light. A phone app gives a confidence score based on a photograph and no smell, no spore print and no look at the stem base. A friend who has picked for years may have been picking the wrong thing for years.
Use three independent lines of evidence before anything is eaten: the physical features against a reliable key, a spore print, and confirmation from someone who genuinely knows the species in the flesh.
The checks that catch the killers
Dig the whole mushroom up, including the base of the stem. Volvas hide underground and they are the Amanita giveaway.
Take a spore print. It costs you an evening and it separates blewits from webcaps, and Honey Fungus from the Funeral Bell.
Smell it. Yellow Stainer smells of ink. St George's Mushroom smells of flour. These are not decorative details.
Cut it in half from top to bottom. A morel is hollow. A false morel is not.
Note the substrate. A mushroom on wood is a different problem from the same-looking mushroom in grass.
Then still be careful
Try a small amount of any species new to you and wait twenty-four hours. Never eat wild mushrooms raw. Keep one uncooked specimen in the fridge from every meal, so that if anyone becomes ill there is something to identify.
Do not serve foraged mushrooms to children, to pregnant women, or to elderly guests, and do not serve them at all to someone who cannot give informed consent to the risk.
And if in doubt, throw it out. There will be more next week.