Split-brain is when two nodes both believe that they are the leader in a single-leader distributed system. This can lead to conflicting writes at any level, from changes to the same record to re-use of keys. Other systems are likely to have acted on information from one of the leaders in the meantime, leading to inconsistent state across the application.
From Kleppman p. 250, links my own:
In one incident at GitHub, an out-of-date MySQL follower was promoted to leader. The database used an auto-incrementing counter to assign primary keys to new rows, but because the new leader’s counter lagged behind the old leader’s, it reused some primary keys that were previously assigned by the old leader. These primary keys were also used in a Redis store, so the reuse of primary keys resulted in inconsistency between MySQL and Redis, which caused some private data to be disclosed to the wrong users.