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If the user picks an invalid timeline (or the default is invalid) they will not discover it until after the restore is complete and recovery starts. In that case they'll receive a message like this:
FATAL: requested timeline 2 is not a child of this server's history
DETAIL: Latest checkpoint is at 0/7000028 on timeline 1, but in the history of the requested timeline, the server forked off from that timeline at 0/600AA20.
This message generally causes confusion unless one is familiar with it. In this case 1) a standby was promoted creating a new timeline 2) a new backup was made from the primary 3) the new backup was restored but could not follow the new timeline because the backup was made after the new timeline forked off. Since PostgreSQL 12 following the latest timeline has been the default so this error has become common in split brain situations.
Improve pgBackRest to read the history files and provide better error messages. Now this error is thrown before the restore starts:
ERROR: [058]: target timeline 2 forked from backup timeline 1 at 0/600aa20 which is before backup lsn of 0/7000028
HINT: was the target timeline created by accidentally promoting a standby?
HINT: was the target timeline created by testing a restore without --archive-mode=off?
HINT: was the backup made after the target timeline was created?
This saves time since it happens before the restore and gives more information about what has gone wrong.
If the backup timeline is not an ancestor of the target timeline the error message is:
ERROR: [058]: backup timeline 6, lsn 0/4ffffff is not in the history of target timeline B
HINT: was the target timeline created by promoting from a timeline < latest?
This situation should be rare but can happen during complex recovery scenarios where the user is explicitly setting the target time.