Non-deterministic behavior in competitive programming can be frustrating because your solution may work on some test cases and fail on others unpredictably. This issue is often caused by uninitialized variables, where the program uses garbage values. In languages like C++ or C, failing to initialize an array or a variable can result in unpredictable behavior. Another source of non-determinism is the use of floating-point numbers, where precision issues may cause different results on different systems. Ensure that all variables are initialized before use, and avoid floating-point comparisons directly; instead, use an epsilon value for comparison. Finally, make sure random number generation (if used) is properly seeded to avoid inconsistencies. Debugging and fixing non-deterministic behavior ensures that your solution behaves consistently across all test cases.