Have you ever made comments when committing in to source control that you wish you could take back? Perhaps in a rage, you entered "Jimmy's code was a pile of fermenting humus that didn't work. So I fixed it!" Now you realize that Jimmy will see it, your boss is going to see it, and you want to change the comments to something that has a bit more tact. Or maybe your reason is far less malicious: you identified a major bug that you just committed, and you would like to update the comment log to say "Don't use this revision. It has a major bug."
In Subversion, the comments can be updated long after the original commit. Log messages are just a property on the repository revision.
svn propset --revision <REVISION> --revprop <MESSAGE> <URL>
Now your malicious revision comment can be overwritten by:
svn propset --revision 123 --revprop "Fixed issue #17" http://svnserver/myrepos/
But next time, do try to be nice to Jimmy.
The opinions expressed herein are my own personal opinions and do not represent
my employer's view in any way.