I was tidying my Katello 3.4.5 installation, deleting old no longer used repositories, and the first repository deleted fine, but when I tried to delete the next one, it got stuck halfway through the deletion with a Candlepin error about the product referring to the repository I just deleted. This was:

Katello::Errors::CandlepinError: Runtime Error product references content which does not exist: ContentData [id: 1481545184271, name: CentOS-7.3-x86_64-extras, label: Acme_Corp_CentOS_CentOS-7_3-x86_64-extras] at org.candlepin.controller.ProductManager.applyProductChanges:658

Looking at the Candlepin DB I found that this repository was still referred to in cp_content:

e3e38602bba447dba0e2bf398b13b3a0 | 1481545184271 | 2016-12-12 12:19:44.271+00 | 2016-12-12 12:19:44.271+00 | /custom/CentOS/CentOS-7_3-x86_64-extras             |        | Acme_Corp_CentOS_CentOS-7_3-x86_64-extras             |              1 | CentOS-7.3-x86_64-extras             |            |              | yum  | Custom |        |                |      0

but this repository had already been removed from Pulp.

So, it looks like the repository deletion isn't fully completing for some reason.