This means an Remote Monitoring (RMON) register has overflowed (wrapped around).
The situation in which the area where the RMON statistical information is stored, has overflowed (wrapped around) is shown with MAC interface 1 of Sibyte ethernet controller located on that linecard. The wrap around means that the counter has reached its max value and restarted from zero.
The message is normal and that the reason you are seeing the message is because you have service internal turned on.
The service internal command is known to generate additional logging messages. This mode is mainly used by the Cisco Technical Support and DE to further investigate issues on the switch (in engineering mode).
In order to stop these messages, issue the no service internal command in the global configuration mode.
Note: If you disable this mode, it does not affect performance or stability in any way.
%SIBYTE-CFC3-6-SB_RMON_OVRFL: RMON register 20 overflow on mac 1