Multiple times saving/casting when I get a spell to backfire pretty much proves it. The one I've had the trouble with is crafty pixies, to make the buildings a little cheaper. I've done this several times actually, trying to save and refresh if I get it to backfire. But every single time I refresh and cast again it still backfires. If there's a 15% chance of backfire, the odds of getting backfire 8-10 times in a row, on several occasions, should be extremely slim, yes? I've actually never had it finally be successful, just have to wait out the hour when it's too irritating to try reloading again.
What am I missing?