This happens when the CVS update command resulted in change conflicts in RBL files.

MacCVSClient then tries to resolve these conflicts. This automatic RBL conflict resolution is a two-step-process. If not all conflicts can be resolved in the first step, MacCVSClient takes CVS diff information into account when trying to resolve the leftover conflicts.

To be able to do this, MacCVSClient automatically runs a CVS diff for the affected files, analyses the diff output, tries to resolve the conflicts, and then discards the diff information.