Last year as I was setting up, I dropped the back of my sparkleball/PVC spider and it shattered.
I was very sad because I love that spider, but then I remembered that I'd covered a 20" garden basket with lights for another failed project, and with a few adjustments to the spider body, it fit rather nicely. So in that case fail + fail = some reduction in the overall level of fail.
I put my timed fog machine behind the spider, which looked a little flatulent each time it went off. Fortunately, I had small boys around to appreciate the humor of my GIANT GLOWING FARTING SPIDER. They were in hysterics over it.
I tried to make a large pumpkin last weekend, by molding florist wire net (aka chicken wire) and wrapping it with orange lights. This looked way better in my head than in reality. It's the Charlie Brown Christmas Tree equivalent of a pumpkin - sad, lopsided and wobbly. My husband tried to praise it as 'rustic' but he was clearly grasping for adjectives.
My biggest fail to date is the fortune teller machine I started working on (mumble mumble) years ago and have never finished. Here it is when my cat decided to have a seat inside the booth. She's yawning, but doesn't she look tough?
It's an old Barbie styling head and hands set inside the top half of a broken Ikea cabinet. She's hooked up to a sort-of Axworthy-esque setup involving a deer motor on the inside of the cabinet, and fishing line on her head and hands through eyehooks in the ceiling . I got as far as getting the movement working and then life got in the way and I never finished the rest of the cabinet. Maybe I'll get back to her someday.