We noticed (every now and then) that some of our pipelines had issues. It appeared as if some jobs started while a previous pipeline was still ongoing (and the 'Do not allow concurrent builds' check was on).

One of our developers wrote a script to check for this type of failure and got:

PROBLEM: build 29319 started before build 29318 finished! (2017-08-31 13:42:41.952000 <= 2017-08-31 13:47:05.655000)

PROBLEM: build 38584 started before build 38583 finished! (2017-10-25 13:54:31.601000 <= 2017-10-25 15:27:10.468000)

PROBLEM: build 39219 started before build 39218 finished! (2018-01-02 08:01:47.422000 <= 2018-01-02 08:13:07.537000)

PROBLEM: build 40171 started before build 40170 finished! (2018-02-15 14:58:57.194000 <= 2018-02-15 15:35:27.225000)

PROBLEM: build 40356 started before build 40355 finished! (2018-02-23 13:31:02.287000 <= 2018-02-23 14:00:22.545000)

We went over the times our Job DSL seed job ran and noticed a Job DSL seed job run ran every time the issue occurred.
I tried to reproduce it with a simple 'sleep' pipeline, but the issue appears to require a more advanced pipeline (perhaps it only gets triggered when going from one step to another?).