I have a deleted branch which hasn't yet been "cleaned up" by the Orphan Item strategy for my Multibranch Pipeline (which is using whatever the settings are defined by New Pipeline in Blue Ocean).

Apparently I can still "Run" the Pipeline, which results in the following error:

Started by user R. Tyler Croy
ERROR: Could not determine exact tip revision of 48620; falling back to nondeterministic checkout
Checking out hudson.scm.NullSCM into /var/jenkins_home/workspace/canary_48620-C3YVMWJP3DJ2ZZJAR4JIMMDY4E5YU7ZHZ4AFTAAQG65ZWZVXYXTA@script to read Jenkinsfile
ERROR: /var/jenkins_home/workspace/canary_48620-C3YVMWJP3DJ2ZZJAR4JIMMDY4E5YU7ZHZ4AFTAAQG65ZWZVXYXTA@script/Jenkinsfile not found
Finished: FAILURE

The only reason I actually discovered this is because the deleted branch still showed up in the "Favorites" section of my home Activity View, which might be another bug unto itself.