If your synchronization failed and you got an error similar to java.sql.BatchUpdateException: Data truncation, it can have two different root causes.
1) Something has changed on the ServiceNow side. Usually someone made a string field longer than before. For example your ServiceNow admin made incident short_description longer (max length) from 80 to 200. So it means the new incident subjects cannot fit into your varchar(80) in the mirror database. To fix this, either manually alter the mirror table (column) or re-create the whole synchronization and perform an initial data load.
2) ServiceNow does not always respect its own data constraints. E.g. try to synchronize the sys_perspective table and it fails by default because one column in this table is configured to have 40 chars as its max length however the longest string in the column has 125 characters by default. There is no other solution than to manually alter the mirror database table to reflect the real situation.