It's the most common occurance of mushroom poisoning in the UK, and it's so easy to make the mistake
often found in hedgerows and open fields, horse muhrooms are delicious and i've often had a great feed of them, but just as often been caught out.
They look so edible, lovely brown gills and a ringaround the stem, but sometimes all is not as it seems, to tell the difference between edible horse mushrooms and inedible yellow stainers, simply either bruise the cap, or slice the stem
If it stains a vivid yellow chances are it's a yellow stainer, it's hard to see in this picture but this was very yellow when cut.
To make things even more difficult, some horse mushrooms can turn a little yellow, but only slightly, if it's a vivid lemon yellow, best disgard them as you've most likely picked yellow stainers
if in doubt leave it out.