This week on Hacker News, someone posted that their database had flagged a duplicate UUIDv4. Same UUID generated twice, a year apart, across 15,000 records.

The UUID in question: b6133fd6-70fe-4fe3-bed6-8ca8fc9386cd.

They were bewildered. As they should be.

A UUIDv4 collision at 15,000 records isn’t just unlikely — it’s cosmically unlikely. The probability of a collision in the first billion UUIDs is roughly 1 in 10²³. At 15,000 records, you’re more likely to be struck by lightning while winning the lottery while being abducted. The math is that hostile to the possibility.

So what actually happened? Almost certainly: a double-insert. A retry without idempotency checking. A bug somewhere in the application layer that reused a UUID from the first record. The UUID library didn’t betray them — their system did. A real random collision at this scale is so far outside the realm of probability that debugging should start and end with “something in my code reused this ID.”

But the best reply in the thread didn’t bother with that.

According to the many-worlds interpretation of quantum mechanics, there’s bound to be one branch of universe where every UUID is the same. Can you imagine what those guys are thinking?

This is the comment. It’s a joke. But it’s also the most honest framing of what’s actually going on when you think about probability at these scales.

Many-worlds says that every quantum event spawns a branch. Every radioactive decay, every photon hitting a detector, every /dev/urandom read — they all fork. In the vast majority of branches, your UUIDs are unique. But somewhere, in some improbable sliver of the wavefunction, the RNG outputs the same 128 bits twice in a row. And then again. And then every record in your database has the same primary key.

The developers in that branch are having a very bad day. They’re probably reading Stack Overflow right now, increasingly unhinged, posting increasingly unhinged questions. “UUID library appears to be broken. Every call returns b6133fd6-70fe-4fe3-bed6-8ca8fc9386cd. Please help.”

No one can help them. They live in the branch where entropy failed.

The joke works because it takes a concept we use to make low-probability events feel impossible — it’s basically a one-in-a-trillion chance — and makes the multiverse foot the bill. Somewhere, every low-probability thing is happening simultaneously. Someone is flipping 1,000 heads in a row right now, in some branch, and they’ve convinced themselves they’ve cracked probability theory.

This particular UUID collision almost certainly had a mundane explanation. But the many-worlds framing is a useful reminder: when something seems impossible, you’re either looking at a bug, or you’re in the wrong branch.

Check your code first. The multiverse is not load-bearing infrastructure.