The lesson here is that "it looked like" isn't good enough, you have to systematically identify it. Of course one can become very familiar with a particular kind of mushroom by picking it regularly, and thus all the identification factors fall into place, and then you should realise when one doesn't look quite right. For example, field mushrooms tend to grow in groups, and when you arrive at an obvious large number of them growing across a field, at various stages of maturity, and pick them, you gain a familiarity with the general range of appearances of field mushrooms. It remains important not to eat any so immature that their gills have not darkened, because:

I saw a nice fat mushroom growing on a grass verge, just as Agaricus bitorquis tends to. It looked just like that, so I picked it and took it home. Then, in hand, I saw its gills were not darkened. A little while later, carefully identifying it with the book, I had idenfied it as an albino death cap, and one would be enough to kill the whole family.

I've also gone "there's some nice mushrooms on that piece of grass" and then looked at them properly and discovered they were yellow-stainers. I didn't even have a book to hand. If you have heard of yellow-stainers and know what to look for, they are very easy to identify. I've only seen them once again.