Steps to reproduce:

1. Select 'Add item' in folder where there are already items present
2. Name the item
3. Type something into the 'copy from' input box, select some item
4. Clear the input box so that it is in its initial state and says 'Type to autocomplete again'
5. Click OK

Jenkins then surprisingly clones the entire containing folder and starts running every job in it, see attached screen recording.

I inadvertently did this in a folder with a large amount of projects, and it effectively created a denial of service situation for 30 minutes and caused building and deploying of artifacts that were not supposed to be built, quite disconcerting and disruptive.