These are all SFDC errors that block Marketo from fully syncing the data. In many cases, Marketo will try later and the data will sync. But you should investigate the Notifications and list of leads so you can fix the problems to reduce data issues.
- Lock Row - someone else is editing this record. Marketo will resolve normally.
- Field Validation issues - that field has an Validation code of some type and Marketo is sending data that isn't conforming. Find out what it is and fix it on either end.
- Apex CPU Limit - something caused SFDC to use up Apex code cycles (too many Apex triggers) and it can't process the incoming data. This can happen for many reasons, but often because you sent SFDC too many records and SFDC has too many triggers to process.
Thanks Josh for wonderful guidelines.