During the migration of SharePoint 2007 site or list you can get this error "The specified path, file name, or both are too long. The fully qualified file name must be less than 260 characters, and the directory name must be less than 248 characters" during the export operation.
To fix this issue you need to change Export folder path. Source 2k7 agent uses "Migration Manager for SharePoint\Web Interface\Migration 2007\Export" folder to store exported data by default. The agent creates new subfolder there for each job and additional subfolder for each SharePoint container (site or list). If container has a long Title you can get error about "long path".
User Guide contains "Temporary Storage of Exported Content" article which describes procedure step-by-step, how to change default path. I suggest to set path like "DRIVE:\MMSP_Export" (for instance D:\MMSP_Export) to avoid "long path" error during the export in future.