If you have a project with several builds, delete the last one, then go to the new last build (the former penultimate build) and ask for its next build, you get the now-deleted last build back! At least in the API; from the UI, you have a Next Build link which gives a 404. This is a symmetric bug from
JENKINS-16194, and is caused by the abuse of selfReference to mean also "no such build".
While reproducing this in a unit test I found another deletion-related bug: AbstractLazyLoadRunMap.numberOnDisk does not get updated when a build is deleted. The symptom is that after the deletion of the last build, say #2, the project's last build will correctly be reported as #1—but a fresh copy which is not equals to the original. The presence of two copies of the "same" build can cause mayhem.