This information was found on Oracle's Metalink site:
Cause: The command "Alter system archive log current" forces manual archiving of the current log, hence the shadow process on which the command was issued will perform the archiving. It will archive the log file and write messages to the trace/alert using the prefix ARCH.
If automatic archiving is turned on then the archiver process will also try to archive this log file, however it will not be able to acquire the lock "kcrrlt" i.e. the lock to protect multiple arch processes from archiving the same logfile.
The failing process will then write to a trace/alert message that it was unable to archive the logfile.
In Oracle 9i the message in the alert.log is more informative:
ARC1: Beginning to archive log# 2 thrd# 1 seq# 24
ARC1: Unable to archive log# 2 thrd# 1 seq# 24
Log actively being archived by another process.
By setting the parameter log_archive_trace = 127 in the INITsid.ORA file, the archiving steps for both the shadow as well as the archiver process can be traced.
Fix: The command “Alter system archive log current” is commonly used in hot-backup scenarios. The message "Failed to archive" can be ignored, as long as it is followed by a “Completed archiving” message.
Workaround: Do not issue the command "Alter system archive log current".