When a tables has a corrupt first page, a backup on said table is largely ignored. The table in question is not backup up at all, there are no errors logged and mariabackup exists without any errors. Using said incremental backup to recover will end up with said table missing. This might be caused by a disk error or something. Innochecksum will detect the issue but not mariabackup. To reproduce:
Get a byte from the first page of the table at some random position and make a note of the value and then write some other value to that position, here we are assuming a table "t1":
Now run mariabackup. In the target directory there is no .ibd file for the table t1 in the "test" database. The exit status from mariabackup is 0 and there are no errors in the log. One can see that the table t1 is not copied though, beyond the .frm file.
At first I noticed this only for incremental backups, but the situation is the same for full backups.