If archiving fails, i.e., the console log ends as follows (as for example seen in the description of JENKINS-7038):

[snip Ant-based build...]
BUILD SUCCESSFUL
Total time: 4 seconds
[snip...]
Archiving artifacts
ERROR: Failed to archive artifacts: 
[snip stack trace related to some connection failure in publish-over-CIFS code...]
Finished: SUCCESS

then Jenkins (1.409.1 which we used back then) decides this a successful build. However, since only part of the artifacts have been archived, and since (in my particular case) another jobs uses the artifacts of this job, this build is not usable.

Therefore any build for which archiving fails should be marked 'unstable', and perhaps even 'failure'.