When code works locally but fails on an online judge, it’s often due to issues with input/output handling, edge cases, or platform-specific behaviors. Ensure that your code strictly follows the input-output format specified in the problem statement, as subtle differences can lead to mismatches. Sometimes, different compilers or environments handle undefined behavior differently, so double-check if your code has any undefined operations like uninitialized variables. Local tests may not cover all cases, so create varied test cases, especially edge cases, to mimic the online judge environment. In many cases, simplifying input/output or avoiding assumptions about data can help your solution work consistently across platforms.