Oracle-database uses a large number (‘SCN’) as a time-stamp for referential-integrity, backup/restores, patch levels etc etc. When instances are linked the highest SCN number overwrites all others.
Trouble is … a ‘warning limit’ is being reached (three quarters of the actual limit). And when exceeded Oracle-database assumes its corrupt and becomes unstable.
But wait! There’s a ‘fix’, a patch which lets you manually set the SCN back below the soft-limit. BUT this needs to occur on all linked instances within a short period, or they are just updated from the one with the highest number (dowh!).
And Finally … the fix is only available for the latest version of Oracle. So an obscure, forgotten, out-of-date, instance in the back-of-beyond can bring down hundreds of shiny new machines (‘patched’ or not) until upgraded.
‘Oracle SCN management’ then, not a disaster just another ongoing DBA task 🙂