I keep my job's config.xml files in a SVN repository.
Once I messed up the copy of the SVN on the Jenkins master so I decided to run a SVN cleanup. I also checked to option to delete unversioned files in my TortoiseSVN GUI.
I did not keep in mind that the "nextBuildNumber" files of the jobs weren't versioned.
However, when the jobs ran the next time, the executors on which they were started died because of the buildnumber starting again at 1 while it had been higher before.
I think that when the nextBuildNumber file for a job is missing and there still are Build Log folders with higher build numbers, that Jenkins should recalculate the number based upon the highest number (adding +1) of the build log folders.