Merging is a very fragile operation which you should do only after reading
and understanding SVN Book http://svnbook.red-bean.com/
In particular if you're commiting after a merge you should ABSOLUTELY
indicate exactly what you have merged, by putting in Changelog:

Merged with command:
svn merge ....

Indicating FULL commands used to do the merge. SVN in current version is
yet unable to assure not merging the same change twice which might lead
to ex. applying the same diff twice. If you fail to comply w/ above rule
you can expect your change to be reverted w/o warning.

You need to obey the below rules:
0- You do only ONE merge per commit.
1- all Merges should be CLEARLY identified, by RELIGIOUSLY adopting the
   format above.
2- No other commit should have a log starting with "Merge".
3- Branch creation should be clearly identified with "Create ... branch.";
   no other log message may start with "Create ...".
4- A single "Merge..." commit MUST NOT do anything else than MERGE a change
   into a branch (local modifications so that the merge works is OK, but any
   other functionality/comment/whatever addition is PROHIBITED).