If talk about error detection method CRC and CS (Check Sum) both are not 100% reliable.

Let's come to the question,

1. If (data bit + error handling bit ) both are corrupted then the error is called Meaningful error and in this case, None can detect it.

Let's take there are n bits of data and k bits of CRC the possible number of data will be 2^n and CRC set will be 2^k. In real 2^k <<<< 2^n. There is many to one function between them.

2. According to the above explanation, CRC can detect all burst errors of less than the degree of the polynomial.