“It worked for me, so it’ll work for you, too.”
“It didn’t work that one time, so it’s generally not valid.”
People love to generalize from the particular and hide from the general behind the particular.
At the risk of over-generalizing, there will always be an exception. And it doesn’t make sense to argue that the thing that worked once should now apply everywhere, nor that the thing that didn’t work once is therefore totally meaningless.