Most modems try to automatically detect and correct errors caused by noise on a phone line.
Blocks are retransmitted until they come through cleanly. If it has to retransmit too much, it slows things down. A few retransmits per minute is completely normal. You might also get randomly disconnected a lot. If you aren't noticing any delays, pauses, or anything else mentioned in the last paragraph, just ignore the message and don't worry about it.