I guess the title says it really, but there is usually some reason, like trying to start basil too early, once I found it needed proper Summer warmth I was away, 'til I sussed that, failure.
New Zealand spinach, I'm guessing it doesn't like drought
Quinoa, chicory, chervil... I don't know didn't come up, have to try again
Brassicas without spray particularly broccoli especially in bad years
Onions and leeks from seed, winter's too cold and springs too short
Heck, peas just get started before summer knocks em out
Golden bantam sweet corn, just would never pollinate for me
Minnesota 13 corn, got completely eaten by deer, well I got one cob. I had deer eat all my cantaloupe plants one year the night i transplanted them
Iron and clay cowpeas, didn't fruit this far north, other cowpeas no problem
Beets don't like being broadcast
Curculios got all the plums
Late frosts get the peaches
Rabbits ate my peanuts
I've failed at a lot of stuff, but that's okay. Failure is the best teacher. It's all part of it