The Birthday Problem surprises people constantly.

Some business proposes using "day and month of birthday" to uniquely identify people in a really small group of like 50 people. There are like 365 days in year. So it should be 𝘶𝘯𝘭𝘪𝘬𝘦𝘭𝘺 that they'll collide.

Sounds reasonable, right?

Wrong. With just 23 people, you're 50% likely to have a collision.

Humans are terrible at estimating hash collision probability. Always do the math on your key space before assuming it's "unique enough."