Personally i wouldnt be concerned unless it was impacting your internet connection.
When data is sent from the modem to the dslam (up) or from the dslam down to your modem (down), it is verified using a CRC check.
Eg. at one end, the content of the packet data is added up and a value is added onto the end. At the other end, the packet is checked and the data should add up to the same value. If it doesnt, the Cyclic Redundancy Check fails and a retransmit request is sent.
Sent is: John jumped over the cat = 643 But arrives as John jumped on the cat = 634 we have an error So the modem or dslam issues a resend request and if it arrives correctly, then it passes on to the next device in the line.
CRC failures can be caused by many things - usually interference. Interference can come from other broadband lines in the street. So this is why i say if its not causing you an issue, then dont worry about it.