This story involves being contacted by an alien race. Against all odds, they are similar to us, exploratory, and communicate through roughly the same channels.
They hover around our planet for a while, and every once in a while make some sort of signal, but it's mostly garbage. Eventually, though, they make an audio transmission in rough, but understandable English. They want to meet, and share their tech with us.
People rejoice, people are suspicious, etc.
The day comes when an alien shuttle lands on the planet's surface, and everyone gathers, etc. When the door opens, the aliens walk out, and are these hideous giant arachnid-like things. I want them to be as non-human as I can. I'm thinking 8 legs, like a scorpion, with the mouth on the bottom, but on the tail are the eyes and along the tail are some sort of dextrous hands. The mouth is just gaping and slobbering and disgusting. They are hideous and frightening.
But, when they speak, through a translator mechanism, they speak of peace, and of brotherhood.
People, though, flip the hell out. Someone starts shooting at them, and they go back into their ship and leave.
Diplomacy occurs, and eventually they come back down, and they start trying to share their crazy science with us.
The issue, though, is that basically everyone, no matter how forward thinking, just can't work with these things. No one ever really trusts to turn their backs on them, because they're clearly monsters. Newborn babies flip out when they come around; it's just ingrained in our animal brains, that these things are bad.
So, despite them never giving us a reason to distrust them, and attempting to make it work, at some point we try to nuke them because clearly they're trying to do something bad.
They are saddened that no peace could be made, but bear us no ill will, and depart to leave us to ourselves.
TL;DR
Nice aliens come down, but they look like bad alien monsters. We cannot overcome our intrinsic fear of them and screw over our chance at an inter-species alliance with very hospitable allies.