I was reading the tutorial of CF 763D and it seemed like the official solution had a bug: it is vulnerable to hash collisions. I'm relatively new to competitive programming. Is it normal for there to be problems that require solutions that are vulnerable to hash collisions? It seems like a good problem should be one that is provably solvable for 100% of all possible inputs.

The problem is here: http://codeforces.net/problemset/problem/763/D

rng_58 also wrote a blog post on this: http://rng-58.blogspot.com/2017/02/hashing-and-probability-of-collision.html. The solution given there also has a nonzero hash collision probability.