I just got this while I was trying to commit from a trunk directory. Doing svn update from the trunk directory did not solve the error; however, doing svn update from the parent directory (where the .svn directory belongs) did solve the error.
My guess about what happened (a use case among other, there may be multiple reasons for this “svn: E160024: resource out of date; try updating”): along to trunk, there was a branches directory. I pulled a branches/branch-1 into master from GitHub. Doing svn update from the parent directory (that is, the root of my working copy) instead of trunk seems to have done something in branches in addition to trunk. When I tried to commit again, there was no error.
However, as I said above, this is one case among probably many others.
Side note: unlike what someone suggested, I don't believe it's a good idea to play manually in the .svn directory.