In competitive programming, common mistakes include making incorrect assumptions about the problem, such as overlooking special input constraints or edge cases. This can lead to wrong answers even when your solution works for typical inputs. Another common mistake is using an inefficient algorithm that fails on large inputs, resulting in time limit exceeded (TLE) errors. Always analyze the time complexity of your solution before submission. Also, incorrect I/O handling, especially with large datasets, can slow down or cause your program to fail. Carefully reading the problem, considering all edge cases, and testing your solution thoroughly can help avoid these common mistakes.