The recent announcement of a collision for the SHA-1 hash algorithm has caused some consternation:

https://shattered.io/

Here is some background to help put this in perspective.

Cryptographic hash functions can have a variety of properties. The property at issue here is:

"Collision resistance - it is computationally infeasible to find any two distinct inputs x, x' which hash to the same output, i.e., such that h(x) = h(x')."

(Menezes, van Oorchot, and Vanstone, "Handbook of Applied Cryptography", section 9.2.2)