Today Google announced the first public full SHA-1 collision, i.e. the first pair of distinct values that when hashed with the SHA-1 function produce the same digest. This should not come as a surprise – it follows the free-start collisions announced at the end of 2015, and many cryptographers had been anticipating full SHA-1 collisions imminently.
To understand what this means, it helps to look at what happened after collisions were found in the MD5 hash function.