We have jobs with similar names: longishName and longishName_2_3_4
The one without the number at the end is a head build. The with the number is a branch.
After upgrading this week from 1.472 to 1.491 and then subsequently downgrading, we found branch builds populated (subdirectories created under builds) into the head job. We could tell at a glance that there was a problem since we use the formatted version number as the build display name.
Also, the head build job was on build # 13, but the build history contained builds with numbers in the 40's and 60's - although not everything from 40-70, just a frightening, confusing handful.
The copied builds still existed, unmolested, in their actual parent jobs.