Hello, hope you are all well. As a fellow Codeforcer, I hate the feeling when my code gets hacked or stuck on a private test case for a trivial reason. Thus I wanted to create this thread where everyone is encouraged to share some niche case where their code unexpectedly broke :(

I'll start first:

• In the Hello 2025 problem A, my unordered hash structure got TLE hacked. Solution is the use a custom hash function, courtesy of Neal's post.

• In Codeforces Round 996 problem D, I tried printing a double to console in cpp. For large values, doubles defaults to scientific notation (e.g. 1.2345e+7), so I was stuck on that private test case for the whole contest :(. Solution is to cast the result to int or long long before printing.

• I've read somewhere that sqrt and log functions can have precision issues at large values? I've never encountered an error when using them so far though.

I hope my mistakes are helpful to you all :)